Electric cars, unclean at any speed

[NachtRitter]

That's the same part I disagree with.... redpoint5 pointed out one problem... even at peak load, constant rpm, there is no genset available that can give you lower fuel consumption from fuel tank to wheels compared to a pusher. Another problem is that even if you assume constant rpm for the genset, you cannot assume constant load, especially if hooked up directly to the motor controller. There will be times when the car is climbing or accelerating when the genset is in its bsfc sweet spot, when cruising it'll be only part way up the bsfc (between your red and blue lines ), and when not drawing any power at all, the genset will be back there roaring away producing nothing.

In short, 1) the genset loses because it already starts out being less efficient, and 2) it then loses more because it has the same bsfc issues that you describe for a pusher scenario.


Do you have an actual case where the genset comes out ahead? I don't mean zooming in on individual pieces, but rather a complete "apples-apples" system, with a real (or realistic) EV as the control vehicle. I'm just not seeing it.

[jamesqf]

Originally Posted by redpoint5

JThe Volt is a fairly ridiculous car that only serves the traveling needs of very few people. It costs a small fortune to purchase, and gets fairly poor fuel economy when using the ICE. Why lug around an ICE everywhere if you don't intend on using it?

You're wrong on all of that. The $40K price is in line with (or cheaper than) other upscale cars, the fuel economy (37 mpg on IC) is comparable to or better than most other cars of its size. As for meeting the needs of very few people... well, I guess that's why you see so few mid-size sedans on the road, no?

As for why you lug around the IC engine when you don't need it, why lug around a spare tire, when you could save weight by carrying it only on the days when you know you're going to have a flat?

But all that's aside from the point I was trying to make, which is that a hybrid car with an IC engine/generator to drive wheels & recharge the battery does work. Whether the engine/generator is in the car or on a trailer is irrelevant to that.

[jamesqf]

Originally Posted by NachtRitter

Another problem is that even if you assume constant rpm for the genset, you cannot assume constant load, especially if hooked up directly to the motor controller. There will be times when the car is climbing or accelerating when the genset is in its bsfc sweet spot, when cruising it'll be only part way up the bsfc (between your red and blue lines ), and when not drawing any power at all, the genset will be back there roaring away producing nothing.

Wrong. A properly designed genset would either be running at optimum BSFC, or not at all. If it's producing more power than the car can use at any moment, the excess power is diverted to the battery (and if the car needs more power, it comes from the battery). If the battery is above a particular charge level, the generator shuts down.

[NachtRitter]

Originally Posted by jamesqf

Wrong. A properly designed genset would either be running at optimum BSFC, or not at all. If it's producing more power than the car can use at any moment, the excess power is diverted to the battery (and if the car needs more power, it comes from the battery). If the battery is above a particular charge level, the generator shuts down.

Fine, you can argue that, but now you're opening the door to a properly designed pusher, which could also be designed to run at or near optimum BSFC (using CVT) while pushing and shut off otherwise. And there's no way to validate one vs the other because neither exists from which to pull real numbers. As well, you've removed any "plug&play" ability with the genset... the EV must be designed to work with the genset, whereas the "properly designed" pusher is self-contained and would work with any EV (well, any EV that has a hitch).

But lets go down that path a little bit... a "properly designed" genset where we use the Leaf as the sample EV... Since the Leaf isn't designed to be charged and driven simultaneously from the factory, I would assume that modifying the entire BEV system to handle the capability you describe would be a significant undertaking... but let's say it's possible. So what size genset are we talking about...? Large enough to offset the constant consumption of the EV while traveling at 65mph? Then you're talking at least 20kW... that already consumes more than the pusher in gph, even assuming it is running at it's peak efficiency. Or smaller, so that some of the EV consumption is coming from the pack and some from the generator? Even a 15kW generator is consuming 1.6gph, more than the unmodified pusher under the same conditions, and then the battery pack still limits your range.

EDIT: It's not until you get down to a 12.5kW diesel generator that you get to a consumption rate that is equivalent to the unmodified pusher (genset consumes ~0.9gph)... with that, you may be able to roughly double the range of the EV but that's not all that impressive with the Leaf's original battery pack range of 75 miles... and we still haven't accounted for the additional load that the trailer puts on the EV when accelerating or climbing... yes, it's partially offset by the regen, but since regen is lossy, not entirely.

So even such a magical "properly designed" genset is not able to outperform the unmodified pusher... if it's already losing against an unmodified pusher, I wouldn't expect it to compare favorably at all to a magical "properly designed" pusher.

Again, if you have a real world example where you can show that the genset outperforms the pusher from an efficiency standpoint, I'd really like to see it. The focused "corrections" aren't really doing it. "Unicorn" scenarios aren't going to do it either, especially since similar unicorn solutions can be applied to both sides, and nobody can validate the solutions with any real world numbers... so there's really no point in going down those paths.

[IamIan]

Originally Posted by redpoint5

As has been discussed, we would need to see real world examples of the setup to get real world numbers. Perhaps in the real world it performs adequately for typical driving and rest stops.

Sorry ... I don't have the spare time or money ... or interest to be honest ... to build it ... just as some vain effort to convince others ... The data and evidence on paper are clear ... if other wish to disprove what has been presented ... that's their nickle.

That having been said ... if I ever win the lottery ... even though I don't play ... then sure when I have lots of spare time and money... I'll build the 40+% efficient fuel to wheel generator trailer... but given the odds of that kind of wind fall ... Don't ever expect it to actually happen.

Originally Posted by redpoint5

On paper though, it's not looking good.

I disagree.
On paper ... as has been described ... and referenced to real devices ... and shown ... it looks very good for the generator in some situations ... and very good for the pusher in other situations.

Originally Posted by redpoint5

Perhaps you can link to a genset that has a sufficiently high kWh to fuel consumption rate that exceeds the 30mpg rate assuming a typical Leaf consumption and speed?

mpg is a very poor efficiency metric.
1 mile is not a set amount of joules ... but 1 gallon is.
You could be at the same % efficiency and at very different MPG , as the energy use per mile changes... heck your efficiency could be higher and have a lower MPG.

Asking for a link to a off the shelf generator ...that you want to compare to the custom eco-modified pusher ... is not a apples to apples comparison.

If you wanted apples to apples ... the pusher would have to be just as off the shelf available as the generator ... and if you did that ... the generator will win the availability by a long margin.

If you instead go back to the back Eco-Modded Generator vs Eco-Modded Trailer ... that both use off the shelf components type of Apples to Apples ... that is back to the example already at hand ... and it results in areas weather either one is better depending on the situation ... in some situations the pusher is more efficient and in other situations the generator is more efficient.

Originally Posted by NachtRitter

In short, 1) the genset loses because it already starts out being less efficient, and 2) it then loses more because it has the same bsfc issues that you describe for a pusher scenario.

#1> less peak efficient ... yes ... other than that ... it depends on the situation... There are many points on the BSFC that have been shown would be more efficient via the generator.

#2> No ... It doesn't have the same BSFC issues the pusher has.

The generator can run at any peak efficiency RPM it wants to , no matter what the vehicle speed is ... the pusher can't do that ... transmissions have limits ... the generator can run at any peak efficiency torque load it wants to , no matter what the torque load at the tire is ... the pusher can't do that... again transmission have limits... and the generator can run at any peak efficiency combined RPM and Toque and Power it wants to ... the pusher can't do that ... again transmissions have limits... the cycle through the batteries while lower ... is still more efficient than some of the pusher BSFC operating conditions ... which gives another functional difference.

They are not the same.

Thus the areas of on the BSFC I color coded... there are situations where the pusher is more efficient than a generator ... and there are situations where it is less efficient than a generator.

Originally Posted by NachtRitter

Do you have an actual case where the genset comes out ahead? I don't mean zooming in on individual pieces, but rather a complete "apples-apples" system, with a real (or realistic) EV as the control vehicle. I'm just not seeing it.

AFAIK , I already listed that.

I did not include any specific vehicle ... because it doesn't matter ... 1 joule of mechanical energy to move vehicle A will move vehicle A the same no matter what it's source is ... 1 joule is 1 joule.

If you disagree about some part of the previous example at hand ... tell me what that part is.

In my previously posted example ... we used the same ICE for both the pusher and the generator ... which I think is a better apples to apples comparison than mixing different ICEs.

But Otherwise ... without knowing what part you disagree with ... Without evidence or such to counter it ... My position stands as supported by the evidence presented to date as:

Originally Posted by IamIan

Which one is more net efficient , the pusher or the generator ... will depend on the situation ... In some situations the pusher will be more net efficient ... in other different situations the generator will be more net efficient.

- - - - - -

Notes about About the ~6.6kw charging limit brought up before about the leaf ... how that effects a range increase is only a factor of the magnitude of that kw of power.

#1> ~6.6kw of pusher power to the wheels will be just as lacking as ~6.6kw of generator power to the wheels.

#2> The ~6.6kw is only in reference to the Leafs slower mode AC-DC on board charge converter ... The Leaf is functionally capable of High power ~60kw charge rates via a 480V 125Amp CHAdeMO compliant fast charging protocol system ... which if you use that system to connect the generator ~60kw is more than enough power.

#3> The charge rates of the Leaf to the batteries only matter for the amount of generator surplus produced beyond what the vehicle is using in real time ... if you only need 10kw real time , but are producing 12kw at that moment ... than 2kw is all the batteries are getting charged with... even though you are running a 12kw generator.

#4> Keep in mind I doubt the Leaf system would OEM just let you do this ... it would likely require at least some level of modding... it is not OEM designed for generator trailer.

[IamIan]

Originally Posted by NachtRitter

Fine, you can argue that, but now you're opening the door to a properly designed pusher, which could also be designed to run at or near optimum BSFC (using CVT) while pushing and shut off otherwise.

Except ... the CVT has it's limits... and this is known.

#1> It can not give the pusher's ICE any RPM at any tire RPM.
#2> It can not give the Pusher ICE any torque at any tire torque.
#3> It can not give the pusher ICE any combined power ( rpm and torque ) at any tire power ( combined rpm and torque ).
#4> The Pusher Trailer has a less efficient cycle efficiency through the batteries than the generator trailer does... if that is even ever used.

that is the point I made on the BSFC ... if the situation means the limits of the transmission must force the Pusher's ICE into or out of those areas ... then that determines if it is more efficient or less efficient than the generator.

If the situation allows for the pusher ICE to stay inside the red ... it is more efficient.

If the situation does not allow for that than it doesn't ... between red and blue ... generator is more efficient for joules not cycled through batteries ... pusher is more efficient for the joules cycled through the batteries.

If the Pusher ICE is ever operated outside the blue ... than it is operating at a lower net efficiency than the generator allows for.

- - - - -

If it helps any to try and see when those situations might happen.

Here are some examples I found for CVT Ratio limits: (Link)

Nissan:
Highest: 0.394:1
Lowest: 2.349:1

Honda:
Highest: 0.452:1
Lowest: 2.427:1

GM:
Highest: 0.443:1
Lowest: 2.618:1

Ford:
Highest: 0.41:1
Lowest: 2.47:1

Chrysler:
Highest: 0.394:1
Lowest: 2.349:1

[sheepdog 44]

Also, a typical belt CVT takes about a 3-5% efficiency hit compared to a manual transmission. On top of that, the 1:1 gear ratio of a manual transmission is the most efficient than any other gear ratio at around 98% efficient (going from memory).

Remember where talking range extended EV's, not range extended Serial and Parallel hybrids (Chevy Volt / Plugin Prius). A pusher or a genset is something that you ideally would rarely use on an EV, so what costs the least is likely the best option.

A genset is cheapest and far lighter cause it excludes drive components, and added rolling resistance. It also has the ability to be removable. A genset that cannot provide 100% of the power needed at all times is a better option. Like the 600cc iBMW range extender that doubles the 100mile EV range to 200miles before it needs to be plugged in. The smaller your engine, the cheaper, the lighter, the easier to make removable from the EV. Otherwise you have a heavy Volt that always needs to carry the dead weight. If you had to build a business model on a pusher trailer or a carry on/ towable generator, The generator would win hands down.

Another point is that the pusher needs to be scaled large enough for peak horsepower (for trailer and vehicle), unlike a smaller genset which only needs to provide average power over a period of time. And with traffic included, that average can be lower still. Generally an engine that must provide peak power has a larger displacement, and thus is less efficient at a low power cruise.

Now you can argue that the Plugin Prius model is the most efficient system of using gasoline to move a car. And i would agree. But the Plugin Prius is not an EV, because it can run only on gasoline after 20 miles and potentially all the time if the user never charges. At that point it's just an efficient hybrid that burns gas, which defeats the purpose of having a clean emission free battery Electric vehicle in the first place. Which begs the point that engine to wheels is incompatible with EV's for many of the above stated reasons despite it's efficiency. Adding a pusher or genset to a 100% EV introduces a whole host of added compromises, which the genset mitigates for the limited intended use.

If the Plugin Prius (pusher) model is used correctly, it is the most efficient use of battery and gas resources. Having both a full EV range and a prius drive, would result in an expensive and heavy, and thus very inefficient version of the Volt. Which makes the smaller scale Prius pusher model almost an argument against the colossal waste that is the 100 mile range battery!

[oil pan 4]

I figure intentionally under sizing, or sizing it to your average power consumption would be best.
That way it can run flat out staying near its best fuel consumption per KWH any time you are going faster than say 55 to 60mph.
Then when you pull into town, stop or otherwise slow down the gen set would be putting out less power in theory, but the natural under size of the gen set would mean that its not too far out side its best FE/watt production even when you are sitting at a stop light.

Best of all you can turn off the gen set.
Problem I see is a lot of FWD vehicles do not recommend flat towing, which is what it would amount to if you tried to shut off your FWD drive train based push trailer and keep driving under electric power.

What I have observed over the years with diesel generators is if you compare running them in stand by or nearly no load to running them full load fuel consumption doubles.
Then if you run them at half load fuel consumption typically goes up by about 1/3 over running idle at speed or under very light load.

[NachtRitter]

Originally Posted by IamIan

mpg is a very poor efficiency metric.
1 mile is not a set amount of joules ... but 1 gallon is.
You could be at the same % efficiency and at very different MPG , as the energy use per mile changes... heck your efficiency could be higher and have a lower MPG.

Honestly, I have no idea what you are trying to say here. A Joule is a measure of energy. A kWH is a measure of energy. A gallon of fuel contains a known measure of energy (135.74MJ/gallon for diesel fuel, according to Diesel fuel - Wikipedia, the free encyclopedia) and therefore could be considered to be a reasonable proxy for a measure of energy. Regardless of the measure of energy we are using, we are talking about trying to travel some distance per hour; 65 miles in the examples I've used. Some amount of energy is going to be consumed in those 65 miles in order to cover them. If you want to convert the gallons of fuel to Joules and call it miles per Joules, or convert it to kWH and call it miles per kWH makes no difference. Miles per gallon works quite well because in both the case of the generator and the pusher, that's what you are starting with... gallons of fuel put into the tank of either option. If you can come up with a better measure of energy used over distance, please do and explain why that is better than MPG.

Originally Posted by IamIan

Asking for a link to a off the shelf generator ...that you want to compare to the custom eco-modified pusher ... is not a apples to apples comparison.

If you wanted apples to apples ... the pusher would have to be just as off the shelf available as the generator ... and if you did that ... the generator will win the availability by a long margin.

If you instead go back to the back Eco-Modded Generator vs Eco-Modded Trailer ... that both use off the shelf components type of Apples to Apples ... that is back to the example already at hand ... and it results in areas weather either one is better depending on the situation ... in some situations the pusher is more efficient and in other situations the generator is more efficient.

There is no entirely off-the-shelf solution for either option. In the case of the generator, you will need to customize the EV to 1) allow charging while driving, and/or 2) provide a higher capacity built-in charger if you want the genset to offset the EV's consumption, and/or 3) customize the entire BEV system in order to direct-drive the motors while also charging the batteries in order to keep the generator's load constant. In the case of the pusher, all the customization is constrained to the pusher itself with the exception of the hitch on the EV (this obviously applies to the generator option too) and maybe the control wiring for the pusher (though that can just as easily be portable).

Originally Posted by IamIan

#1> less peak efficient ... yes ... other than that ... it depends on the situation... There are many points on the BSFC that have been shown would be more efficient via the generator.

#2> No ... It doesn't have the same BSFC issues the pusher has.

The generator can run at any peak efficiency RPM it wants to , no matter what the vehicle speed is ... the pusher can't do that ... transmissions have limits ... the generator can run at any peak efficiency torque load it wants to , no matter what the torque load at the tire is ... the pusher can't do that... again transmission have limits... and the generator can run at any peak efficiency combined RPM and Toque and Power it wants to ... the pusher can't do that ... again transmissions have limits... the cycle through the batteries while lower ... is still more efficient than some of the pusher BSFC operating conditions ... which gives another functional difference.

They are not the same.

Thus the areas of on the BSFC I color coded... there are situations where the pusher is more efficient than a generator ... and there are situations where it is less efficient than a generator.

Here, I'm not clear if we're talking about the same thing. You're saying that the generator can run at peak efficiency more often than the pusher can run at peak efficiency, right? To me that doesn't mean anything because the peak efficiency of a generator may not be as good as the peak efficiency of a pusher... that is, the generator BSFC != pusher BSFC. To me, the more efficient solution is the one that gets me farther on a gallon of diesel, and as I've shown, the diesel generators which can keep up with the EV's energy consumption at 65mph will get me only a fraction of the distance on a gallon of gasoline compared to the pusher.

Even if we where to assume that the exact same diesel engine, the VW TDI engine, is being used for the pusher as for the generator, I really do not know that the TDI has enough horsepower to run a 20+ kW electric generator, or that the TDI's peak efficiency RPM aligns with the electric generator's peak output rpm. For example, a 2-pole generator must run at 3600RPM to output 60Hz; a 4-pole generator must run at 1800RPM to output 60Hz. Neither of those RPMs fall in the TDI's peak efficiency spot on the BSFC chart.

We also have to be talking about the same range extending capabilities for the generator as the pusher... That is, the pusher allows unlimited range without extended stopping provided the driver can fill up the tank with more fuel. In order to compare apples to apples, the generator then also has to provide that same capability... otherwise we'll start going off into la-la land where we could also downsize the pusher to only provide a fraction of the total cruising hp required to maintain our goal of 65mph... etc.

Therefore, the generator must replenish the energy consumed by the EV, Joule for Joule, in which case we are talking about a 20kW generator if used with a middle-of-the-road (from EV efficiency point of view) Leaf, right? In which case:

1. As noted, even at peak efficiency, constant load, a typical off-the-shelf 20kW will consume more gph than the TDI pusher running at 65mph... In this situation, the genset looses. If you change the parameters of our situation (travel unlimited miles at 45mph instead of 65mph), you may be able to downsize the genset but at the same time the pusher will get even better fuel economy... so even in the situation of changing parameters, the genset looses.
2. If you hook the genset directly to the motors (without the super-fancy customization mentioned by jamesqf) in order to bypass the battery charging losses, your genset load will vary (e.g. constant RPM, but varying torque) as the electric motor's load varies... yes, that's effectively a direct genset to wheels connection, and the genset will not be at it's peak efficiency all the time. Plus you'll need to up-size the genset in order to accommodate the times that the power draw is greater than the draw during cruising. In this situation, the genset looses.
3. If you manage to do the full customization proposed by jamesqf where the genset powers the electric motors and charges the battery in such a way that the genset is under constant load all the time, then you're effectively back to a very complex form of #1... the genset must be sized large enough that the EV's consumption is offset and the battery isn't drained regardless of route. So we're still talking about a genset that is around 20kW, which we've already established consumes more than the pusher per mile traveled.

Originally Posted by IamIan

AFAIK , I already listed that.

I did not include any specific vehicle ... because it doesn't matter ... 1 joule of mechanical energy to move vehicle A will move vehicle A the same no matter what it's source is ... 1 joule is 1 joule.

All I saw is that you pointed to a BSFC chart for a TDI... Not a BSFC chart for a generator (one which can compensate for the power consumed by the EV), and not a comparison of BSFC between the TDI and the generator. Generally, you can't run a generator at any RPM you choose; you have to run it at a specific RPM to get the specific AC frequency which means that the BSFC chart for an automotive engine is likely invalid for a generator engine. As well, just looking at the BSFC chart does not tell the whole story... in order to get the whole story of how the gallon of fuel that you put into the generator (the energy input) gets converted to some distance traveled at 65mph (the energy output), you must include the vehicle... because that absolutely does matter. It's the same with the pusher... that same gallon of fuel will generate some amount of power, and how efficiently that power gets converted to some distance traveled at 65mph is entirely dependent on the rest of the system (transmission, final drive, tires, etc); ignoring the rest of the system makes the knowledge of how much power the pusher engine creates entirely useless information.

On the other hand, I looked at how far each solution can take me on a gallon of fuel, and found no situation where a generator will take me further than a pusher... even assuming no efficiency-related modifications on the pusher and genset-specific modifications to the EV (such as modifying the charger to accept a 20kW charge vs 6.6kW). If you could point out any flaws in my approach, please do.

Originally Posted by IamIan

If you disagree about some part of the previous example at hand ... tell me what that part is.

In my previously posted example ... we used the same ICE for both the pusher and the generator ... which I think is a better apples to apples comparison than mixing different ICEs.

1. I don't believe we know that the same ICE even has the ability to run an electric generator with sufficient output to offset the consumption of our sample EV at 65mph. I searched and found no examples of a 4cyl TDI being used in this way.
2. Electrical generators must run at specific RPMs in order to get specific frequency outputs... 1800RPM is the closest but is still not in the peak efficiency portion of the BSFC chart.
3. The BSFC chart does not tell the whole story... the entire path from ICE to road must be considered. While I agree that the EV components are relatively efficient, the currently available mass-produced EVs are not efficient enough in converting power from the plug to the wheels to allow a (currently available mass-produced) generator to outperform a pusher based on a (commonly available mass-produced) TDI front end from an efficiency perspective... in any situation.

Originally Posted by IamIan

But Otherwise ... without knowing what part you disagree with ... Without evidence or such to counter it ... My position stands as supported by the evidence presented to date as:
- - - - - -
Notes about About the ~6.6kw charging limit brought up before about the leaf ... how that effects a range increase is only a factor of the magnitude of that kw of power.

#1> ~6.6kw of pusher power to the wheels will be just as lacking as ~6.6kw of generator power to the wheels.

I haven't researched that, but it is a valid point. However, even in this case I do not believe we can assume that the genset wins.

#2> The ~6.6kw is only in reference to the Leafs slower mode AC-DC on board charge converter ... The Leaf is functionally capable of High power ~60kw charge rates via a 480V 125Amp CHAdeMO compliant fast charging protocol system ... which if you use that system to connect the generator ~60kw is more than enough power.

Yes, I mentioned that, and now we're talking about a generator that is twice as heavy as the pusher, consumes over two times as much fuel per mile, and costs in the range of 10x as much. In my description, I pointed out that even if such a generator only runs for ~30 minutes every hour during the 65mph travel, it's still consuming more fuel per mile than the pusher. And at the same time, you are shortening the life of your battery pack, thereby adding even more cost per mile... so once again, in this situation the genset looses.

#3> The charge rates of the Leaf to the batteries only matter for the amount of generator surplus produced beyond what the vehicle is using in real time ... if you only need 10kw real time , but are producing 12kw at that moment ... than 2kw is all the batteries are getting charged with... even though you are running a 12kw generator.

Errr... I'm not sure I get your point... if you need 14kW real time and have only a 12kW generator, then you're drawing an extra 2kW from the batteries... so what? As I've pointed out many times, the Leaf consumes ~20kWH every hour (on average) when traveling at 65mph. If that's our baseline, and our goal is to extend the range of the EV as far as we want to go, then the generator must be sized to at least offset the consumption... so it has to be at least 20kW. A 20kW generator consumes more fuel per mile than the pusher... so in this situation, the genset still looses.

#4> Keep in mind I doubt the Leaf system would OEM just let you do this ... it would likely require at least some level of modding... it is not OEM designed for generator trailer.

Exactly... there is no off-the-shelf solution regardless of which approach is being taken.

So, still no situation in sight where the genset outperforms the pusher from an efficiency perspective. Pointing to the TDI BSFC chart doesn't give us a fuel tank to miles traveled number for either the pusher or the generator. Saying that the pusher is "less efficient outside the blue area" of the BSFC chart doesn't tell me anything without knowing how that translates to number of miles traveled for that gallon of fuel... and that would be based on what percentage of time (or miles) you expect the pusher to be outside the blue area... and how that compares to a genset approach under the same conditions.

I ask again... is there a real world scenario you can provide where a genset outperforms a pusher? Painting lines on a BSFC chart does not give the whole picture, so pointing to it again won't answer the question.

[NachtRitter]

Originally Posted by oil pan 4

I figure intentionally under sizing, or sizing it to your average power consumption would be best.
That way it can run flat out staying near its best fuel consumption per KWH any time you are going faster than say 55 to 60mph.
Then when you pull into town, stop or otherwise slow down the gen set would be putting out less power in theory, but the natural under size of the gen set would mean that its not too far out side its best FE/watt production even when you are sitting at a stop light.

Correct, you can undersize the genset... but that will limit your range and therefore falls outside our assumptions. Technically, you could undersize the pusher also, and then we'd be down a different path altogether because we'd have to agree on how far we want to go (200 miles? 400 miles?) in order to properly compare one to the other.

Originally Posted by oil pan 4

Best of all you can turn off the gen set.
Problem I see is a lot of FWD vehicles do not recommend flat towing, which is what it would amount to if you tried to shut off your FWD drive train based push trailer and keep driving under electric power.

Should be able to turn the pusher off too... and yes, the Mk4 VWs with manual transmissions may be flat towed. Might actually be interesting to have the axles set up like a bicycle freewheel... only engage when the engine spins the axles forward... not saying that's possible, but it would be cool.

Originally Posted by oil pan 4

What I have observed over the years with diesel generators is if you compare running them in stand by or nearly no load to running them full load fuel consumption doubles.
Then if you run them at half load fuel consumption typically goes up by about 1/3 over running idle at speed or under very light load.

The generator site that I've been linking to provides some consumption charts from no load to full load... most of them are relatively linear. Of course if you're talking about only partly loading a generator, then you'd need to get an oversized generator rather than an undersized one, and then you'd probably start noticing that you're not gaining anything from a fuel consumption perspective for a given kW output.