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NachtRitter · 07-16-2013, 01:58 PM

Well, I did some research (not knowing much about controller, motor, etc efficiencies) and came up with something that I originally thought would bolster the position of the genset trailer approach, but then I checked my maths and discovered a major flaw in my thinking...

I'll summarize what I learned here for the benefit of others (sorry Frank; this will be a long one!).

EV Details:
The EV I picked as the representative travel vehicle is the 2013 Nissan Leaf with the optional 6.6kW charger. It's not the most efficient commercial EV out there, nor the least. I have no idea if a Nissan Leaf would be able to tow anything, but for the sake of this discussion, let's assume that it does have the ability to tow whatever we decide to put behind it for the range extender.

Official (aka EPA) specs for the 2013 Nissan Leaf are 102 MPGe highway (consuming 33kWH/100 miles or 0.33kWH/mile) and 129 MPGe city (consuming 26kWH/100 miles or 0.26kWH/mile) (see Compare Side-by-Side) The MPGe figure takes into account all the losses from the wall plug to the wheels (see Miles per gallon gasoline equivalent - Wikipedia, the free encyclopedia), so I don't have to know the details of the charger, controller, motor, etc. efficiencies. Stated range with a full charge is about 75 miles at 65mph.

(Some more info on the Leaf: http://en.wikipedia.org/wiki/Nissan_Leaf)

We all know that the EPA testing protocols don't reflect real world driving as well as we'd like, so I also looked for real user's consumption and found data for the earlier (and less efficient) 2012 Leaf... surprisingly, at a steady-state 65mph cruise, users tended to achieve even lower consumption... 0.278kWH/mile (My Nissan Leaf Forum - Range Chart) and even as low as 0.25kWH/mile. Of course, this assumes a battery pack with no degradation. So we could assume a worst case of 0.33kWH/mile for the consumption.

I don't know what the impact on EV efficiency would be by pulling a trailer, so I didn't take that into account. For my end conclusion, it doesn't matter anyway.

Pusher details:
For the pusher, I assumed it would be a relatively modern TDI as was implied, for instance a Mk4 Golf or Jetta front end with the ALH TDI, with no specific engine modifications other than obvious ones such as removing the power steering pump and AC pulley and potentially replacing the alternator with another that has higher output. I'm assuming this just so we can use known data rather than guessing at how the engine modifications would change the output.

I picked the ALH just because I am somewhat familiar with it; there may be newer TDIs that are more efficient, but it's hard to tell since the newer cars seem to have worse FE... I know some of that is due to the updated emmissions equipment and some is due to the higher weight of the newer cars.

Given a Mk4 front end as the pusher power source, I believe that limits the transimission choices to either a 5MT or an auto transmission. I suppose it would be possible to fab up an adapter in order to attach a DSG or even a CVT to the engine, along with whatever work is needed to also control those adaptations, but for me this level of modification is beyond what I imagine to be reasonable. Let's assume that it's possible to at least fab up a way to control the 5MT from the EV cabin, maybe using only the top gear to reduce the complexity, and the EV itself would take care of getting the combination up to the speed necessary to engage the gear on the pusher.

Let's also assume that the 5th gear has been swapped so that the engine runs at (or very near) an optimal RPM when it happens to get loaded (e.g. pushing up a grade) at 65mph.

For a stock ALH TDI Golf driving at 65mph, 55MPG is achievable (according to MPG at MPH chart - TDIClub Forums). This comes out to 1.18gph fuel consumption for steady state cruise (not accounting for acceleration or up-grade pushing). With my assumptions, the 5th gear ratio has been optimized for 65mph steady state but we've also added a trailer which likely will increase the aero and rolling drag, so it's probably a wash. Potentially the driver can actively P&G as well as EOC the pusher, maybe averaging as high as 0.93gph (aka 70mpg at 65mph) for the combination of EV and trailer.

As with the MPGe figure, the MPG (and the derived gph) figure takes into account all the losses from the fuel that has been pumped into the tank to the wheels turning on the pavement.

Genset details:
Knowing that the maximum charge that the Leaf can take is 6.6kW, I will assume the genset would be sized appropriately for that. Gensets are rated at gph at a particular load, which (just like MPGe and MPG) takes into account all the losses of the unit from tank to kW output.

As an example, the Isuzu 8kW diesel generator (see http://www.generatorsales.com/order/...asp?page=800TS) is sized to provide 8kW continuous, 240V @ 33 Amps, and under max load consumes 0.82gph. The Nissan Leaf (as mentioned) will take up to 6.6kW, 240V @ 27.5 Amps, which is easily within the capabilities of the generator. In fact, at max battery charging, the Leaf is only using 82.5% of the generators max load, and so the generator only uses 0.68gph (according to the chart given at the link, the fuel use appears to be linear between full load and 3/4 load).

Proof that the genset is more efficient!
"Ah ha!" I thought. An hourly consumption of 0.68gal is clearly better than an hourly consumption of 0.93gal! See, genset wins!!

But wait...
Then I checked my maths...

If the Leaf is consuming a worst case of 0.33kWH/mile at 65mph, then in one hour of travel, it would consume 21.45kWH. Best possible case of 0.25kWH/mile (which is probably not realistic with a trailer) means a consumption of 16.25kWH each hour. Ack! The genset can only replenish 6.6kWH each hour, max! So while the genset does have the ability to extend the range of the EV, it's not going to extend it enough to make a 600 or 800 mile trip feasible. The BEST possible range, assuming the BEST possible consumption figures is about 124 miles before the battery pack is down to 10% at which point the driver must stop in order to recharge to avoid hurting the battery pack (assumptions: 24kWH non-degraded battery pack, 0.25kWH/mile consumption, 6.6kW constant charging while driving). Worst case is (at 0.33kWH/mile) is about 88 miles.

So even in the best possible case, the genset will only allow about 2 hours of driving (vs 1 hour without the extender) at which point you'd have to stop and recharge. The 6.6kW charger can charge to full in about 4 hours, or if a DC "Fast Charger" happens to be available along your route, you could recharge in about 30 minutes. However, the DC Fast Charger (rated at 60kW!!) will end up degrading the battery pack faster, plus they are pretty hard to find (as I understand it) so I wouldn't consider that to be a viable option.

I suppose you could also do a 60kW genset to replicate the Fast Charger, but a genset of that capability would be freakin' expensive, heavy, and obviously would consume a great deal more gph. Even an efficient one (such as this one: http://www.generatorsales.com/order/...page=Perkins60), which is set to only run 20 minutes an hour (to quickly top up the battery pack) would consume 1.14gph, but it's over 2,000 lbs all by itself, not considering a trailer that would have to support that weight... probably not a realistic option.

Another approach is to slow down so that the EV doesn't consume quite as much per hour as it does at 65mph. But of course that can be done with the pusher as well, and while the 6.6kW genset consumes a steady 0.68gph regardless of the speed the EV drives, the pusher consumption improves rapidly the slower it is driven... for example, at worst case for a stock Mk4 Golf with the TDI ALH (per the chart linked above), it should be possible to get about 72mpg at 45mph... that gives 0.625gph... already better than the genset. Yet at 45mph, with the genset range extender, the range would only increase to ~200 miles (about 4 1/2 hours of driving) before requiring a recharge.

Conclusion:
I still agree it's all situational, but after looking at these numbers, I think the situations where a genset makes sense compared to a pusher (with all else being equal) have shrunk down to effectively none from my perspective. If I'm going on a 600, 800, or even 1000 mile trip, I certainly do not want to stop every 2 hours in order to wait 4 hours for a full recharge. Nor do I want to deal with finding the Fast Charger "needle in the haystack" (which may or may not be anywhere near my route, or spaced where I want along my route) to do the 30 minute charge. Nor do I want to attempt to drive the entire way at 45mph or less.

Of course, "in the real world", I wouldn't try to drive an EV that kind of distance, regardless of any range extending options. I'll just take my trusty TDI, which gives me about 65 - 70MPG steady state cruising at 60mph (0.92 - 0.86gph) and be happy with that.

Disclaimer:
Your situation may vary.

07-16-2013, 01:58 PM	#226 (permalink)
NachtRitter NightKnight Join Date: Dec 2008 Location: Placerville, CA Posts: 1,594 RippinRoo - '05 Subaru Legacy Wagon 2.5 GT Subaru 90 day: 21.16 mpg (US) Helga - '00 Volkswagen Jetta TDI TEAM VW AUDI Group Diesel 90 day: 53.91 mpg (US) Olga - '03 Volkswagen Jetta Wagon 90 day: 46.24 mpg (US) Thanks: 303 Thanked 311 Times in 186 Posts	Well, I did some research (not knowing much about controller, motor, etc efficiencies) and came up with something that I originally thought would bolster the position of the genset trailer approach, but then I checked my maths and discovered a major flaw in my thinking... I'll summarize what I learned here for the benefit of others (sorry Frank; this will be a long one!). EV Details: The EV I picked as the representative travel vehicle is the 2013 Nissan Leaf with the optional 6.6kW charger. It's not the most efficient commercial EV out there, nor the least. I have no idea if a Nissan Leaf would be able to tow anything, but for the sake of this discussion, let's assume that it does have the ability to tow whatever we decide to put behind it for the range extender. Official (aka EPA) specs for the 2013 Nissan Leaf are 102 MPGe highway (consuming 33kWH/100 miles or 0.33kWH/mile) and 129 MPGe city (consuming 26kWH/100 miles or 0.26kWH/mile) (see Compare Side-by-Side) The MPGe figure takes into account all the losses from the wall plug to the wheels (see Miles per gallon gasoline equivalent - Wikipedia, the free encyclopedia), so I don't have to know the details of the charger, controller, motor, etc. efficiencies. Stated range with a full charge is about 75 miles at 65mph. (Some more info on the Leaf: http://en.wikipedia.org/wiki/Nissan_Leaf) We all know that the EPA testing protocols don't reflect real world driving as well as we'd like, so I also looked for real user's consumption and found data for the earlier (and less efficient) 2012 Leaf... surprisingly, at a steady-state 65mph cruise, users tended to achieve even lower consumption... 0.278kWH/mile (My Nissan Leaf Forum - Range Chart) and even as low as 0.25kWH/mile. Of course, this assumes a battery pack with no degradation. So we could assume a worst case of 0.33kWH/mile for the consumption. I don't know what the impact on EV efficiency would be by pulling a trailer, so I didn't take that into account. For my end conclusion, it doesn't matter anyway. Pusher details: For the pusher, I assumed it would be a relatively modern TDI as was implied, for instance a Mk4 Golf or Jetta front end with the ALH TDI, with no specific engine modifications other than obvious ones such as removing the power steering pump and AC pulley and potentially replacing the alternator with another that has higher output. I'm assuming this just so we can use known data rather than guessing at how the engine modifications would change the output. I picked the ALH just because I am somewhat familiar with it; there may be newer TDIs that are more efficient, but it's hard to tell since the newer cars seem to have worse FE... I know some of that is due to the updated emmissions equipment and some is due to the higher weight of the newer cars. Given a Mk4 front end as the pusher power source, I believe that limits the transimission choices to either a 5MT or an auto transmission. I suppose it would be possible to fab up an adapter in order to attach a DSG or even a CVT to the engine, along with whatever work is needed to also control those adaptations, but for me this level of modification is beyond what I imagine to be reasonable. Let's assume that it's possible to at least fab up a way to control the 5MT from the EV cabin, maybe using only the top gear to reduce the complexity, and the EV itself would take care of getting the combination up to the speed necessary to engage the gear on the pusher. Let's also assume that the 5th gear has been swapped so that the engine runs at (or very near) an optimal RPM when it happens to get loaded (e.g. pushing up a grade) at 65mph. For a stock ALH TDI Golf driving at 65mph, 55MPG is achievable (according to MPG at MPH chart - TDIClub Forums). This comes out to 1.18gph fuel consumption for steady state cruise (not accounting for acceleration or up-grade pushing). With my assumptions, the 5th gear ratio has been optimized for 65mph steady state but we've also added a trailer which likely will increase the aero and rolling drag, so it's probably a wash. Potentially the driver can actively P&G as well as EOC the pusher, maybe averaging as high as 0.93gph (aka 70mpg at 65mph) for the combination of EV and trailer. As with the MPGe figure, the MPG (and the derived gph) figure takes into account all the losses from the fuel that has been pumped into the tank to the wheels turning on the pavement. Genset details: Knowing that the maximum charge that the Leaf can take is 6.6kW, I will assume the genset would be sized appropriately for that. Gensets are rated at gph at a particular load, which (just like MPGe and MPG) takes into account all the losses of the unit from tank to kW output. As an example, the Isuzu 8kW diesel generator (see http://www.generatorsales.com/order/...asp?page=800TS) is sized to provide 8kW continuous, 240V @ 33 Amps, and under max load consumes 0.82gph. The Nissan Leaf (as mentioned) will take up to 6.6kW, 240V @ 27.5 Amps, which is easily within the capabilities of the generator. In fact, at max battery charging, the Leaf is only using 82.5% of the generators max load, and so the generator only uses 0.68gph (according to the chart given at the link, the fuel use appears to be linear between full load and 3/4 load). Proof that the genset is more efficient! "Ah ha!" I thought. An hourly consumption of 0.68gal is clearly better than an hourly consumption of 0.93gal! See, genset wins!! But wait... Then I checked my maths... If the Leaf is consuming a worst case of 0.33kWH/mile at 65mph, then in one hour of travel, it would consume 21.45kWH. Best possible case of 0.25kWH/mile (which is probably not realistic with a trailer) means a consumption of 16.25kWH each hour. Ack! The genset can only replenish 6.6kWH each hour, max! So while the genset does have the ability to extend the range of the EV, it's not going to extend it enough to make a 600 or 800 mile trip feasible. The BEST possible range, assuming the BEST possible consumption figures is about 124 miles before the battery pack is down to 10% at which point the driver must stop in order to recharge to avoid hurting the battery pack (assumptions: 24kWH non-degraded battery pack, 0.25kWH/mile consumption, 6.6kW constant charging while driving). Worst case is (at 0.33kWH/mile) is about 88 miles. So even in the best possible case, the genset will only allow about 2 hours of driving (vs 1 hour without the extender) at which point you'd have to stop and recharge. The 6.6kW charger can charge to full in about 4 hours, or if a DC "Fast Charger" happens to be available along your route, you could recharge in about 30 minutes. However, the DC Fast Charger (rated at 60kW!!) will end up degrading the battery pack faster, plus they are pretty hard to find (as I understand it) so I wouldn't consider that to be a viable option. I suppose you could also do a 60kW genset to replicate the Fast Charger, but a genset of that capability would be freakin' expensive, heavy, and obviously would consume a great deal more gph. Even an efficient one (such as this one: http://www.generatorsales.com/order/...page=Perkins60), which is set to only run 20 minutes an hour (to quickly top up the battery pack) would consume 1.14gph, but it's over 2,000 lbs all by itself, not considering a trailer that would have to support that weight... probably not a realistic option. Another approach is to slow down so that the EV doesn't consume quite as much per hour as it does at 65mph. But of course that can be done with the pusher as well, and while the 6.6kW genset consumes a steady 0.68gph regardless of the speed the EV drives, the pusher consumption improves rapidly the slower it is driven... for example, at worst case for a stock Mk4 Golf with the TDI ALH (per the chart linked above), it should be possible to get about 72mpg at 45mph... that gives 0.625gph... already better than the genset. Yet at 45mph, with the genset range extender, the range would only increase to ~200 miles (about 4 1/2 hours of driving) before requiring a recharge. Conclusion: I still agree it's all situational, but after looking at these numbers, I think the situations where a genset makes sense compared to a pusher (with all else being equal) have shrunk down to effectively none from my perspective. If I'm going on a 600, 800, or even 1000 mile trip, I certainly do not want to stop every 2 hours in order to wait 4 hours for a full recharge. Nor do I want to deal with finding the Fast Charger "needle in the haystack" (which may or may not be anywhere near my route, or spaced where I want along my route) to do the 30 minute charge. Nor do I want to attempt to drive the entire way at 45mph or less. Of course, "in the real world", I wouldn't try to drive an EV that kind of distance, regardless of any range extending options. I'll just take my trusty TDI, which gives me about 65 - 70MPG steady state cruising at 60mph (0.92 - 0.86gph) and be happy with that. Disclaimer: Your situation may vary. __________________ My VW MkIV Aerodynamic Mods (For Jetta and Golf) thread VW Save as You Drive. Background information for expert fuel savers. Last edited by NachtRitter; 07-16-2013 at 02:07 PM..