Engine efficiency

[Ecky]

Originally Posted by Old Mechanic

Also consider the generally accepted practice of not charging the battery to 100% or discharging it to below 20% and the fuel tank on the Leaf just shrunk by 40%.

Without engine design optimization, the power train is the secret to higher efficiency. 80% mileage improvement is a practical figure for a power train that allows only best BSFC engine operation. It's also possible with current technology to make an engine better than 40% efficient on gas and closer to 50% on diesel if you optimize the engine for only operating at best BSFC instead of using throttling to control power output, or in the case of the diesel super lean burn AF ratios.

regards
Mech

I was under assumption that the Leaf probably already has something like an 80/20 limit in charging, since it generally improves battery life.

Also, while it's true that electric motors do have efficiency maps, the ones I've looked at do not fall off nearly as sharply as on gasoline motors.


I agree that adding lossy conversions is inefficient, but we're looking for the least of evils in real-world conditions.

[Ken Fry]

Originally Posted by Ken Fry

So the larger engine would consume 290/230 or 1.26 times as much fuel.

At lower cruise speeds, the difference would be much greater.

For example: Suppose the car is in town, cruising at 30 mph. The HP used will depend on a lot of factors (parasitic loads, AC on-off, relative effects of rolling resistance vs aero, etc) but an OK guess would be that 4 hp is required in the same car that required 11 hp at 60.

1800 rpm seems like a reasonable rpm for driving around town. 4 hp would require 11.6 lb-ft (15.6 Nm) at 1800 rpm: 500 gr/kWh with the 2000cc engine.

Using the same rpm, with the 500cc engine, the required torque would show up at the 62.4 Nm line (15.6 x 4). At 1800 rpm, we are at about 305 g/kWh.

(This torque scaling is avoided in charts that show BMEP instead of torque, incidentally.)

At this reduced load, the smaller engine is 500/350, or 1.63 times as efficient.

A holistic glance at the chart shows that for both engines, it would be better to drive at 900 rpm, where we'd need double the torque. This improves the efficiency of the bigger engine to maybe 410 gr/kWh. The smaller engine would be at about 260 g/kWh. The comparative ratio is then 410/260 or 1.58 times as efficient for the small engine.

Jack McCornack's car shows the advantage of the combined effects of a small (32hp) engine and a diesel in a car that is of similar mass to yours. He is using both a small (32 HP) engine and a diesel in a small, light car. In his streamlined version, he is getting roughly 80 mpg at around 60, which is pretty impressive.

Because your engine is much larger, and of less efficient technology, you probably can't expect anything very close to Jack's fuel efficiency. However, your solution will cost far less, and will be far cleaner, in terms of NOx, particulates... the whole nine yards. Also, a gallon of diesel is over 8 lbs -- so a gallon of diesel represents more oil. So even if you are down around 65 or 70 mpg (at the same cruise condition that produces 80 mpg in Jack's car) The actual resource consumption (if that matters to you) might not be so large a difference. (As you probably know, this stuff can get quite complicated when you consider the energy invested in the different refining processes, etc etc.)

I understand that you, Doug, are probably familiar with this stuff, but supply this level of detail because others might be just learning about this (contemplating, for example, what size engine to use in the next Vetter challenge) etc. I am a natural pedant, and having taught this stuff, the teacher role pops up. (As a measure of how old I am, when I taught this, 25% was considered pretty good efficiency. The Saturn engine, if the 230 peak is to be believed, is about 36% efficient, using 33.7 kWh per gallon of gas and 6.2 lb.gallon (and the mixed units !! that are required of people in the US these days.)

Regards,
Ken

[Ken Fry]

Originally Posted by mort

... Pedal to the metal at all times!

-mort

Really great summary. You've condensed one of John Heywood's books into a few paragraphs!
Kudos,
Ken

[Ken Fry]

Originally Posted by Old Mechanic

Also consider the generally accepted practice of not charging the battery to 100% or discharging it to below 20% and the fuel tank on the Leaf just shrunk by 40%.

Another well-thought-out post. My only nit pick has mainly to do with the above -- which seems like a non sequitur: the size of the tank (electric or gas) has no direct effect on fuel efficiency. (In both cases, there is of course the indirect effect of weight... but that fits into a discussion of vehicular efficiency rather than one on powerplant efficiency.) But you presented a bunch of good, solid, points.

Thanks,
Ken

[drmiller100]

if the generator is 90 percent efficient, the charge controller loses 5 percent, battery loses 5 percent when charging, motor controller uses 3 percent, motor is 90 percent efficient, batteries cost 15,000 bucks, and you lose 10 percent in the power company's transmission and distribution, your cost per mile eventually starts to add up.

[drmiller100]

Mort, Ken,

Some things I am trying to get my head wrapped around.

Lets say the exhaust back pressure is 5 psi, we are running at super slow RPM, and at very light loads.

Basically, I am trying to describe retarded exhaust cam with huge internal EGR.

What happens to the efficiency math now? Our input temp is higher, but our effective compression ratio is also much higher - in other words our peak temp should be higher.

Next question - lets say we inject fine droplets of liquid water into the intake. Some of it turns to steam, but on compression stroke most of it will turn back to liquid sucking some of the heat of compression. When the burn event happens, heat goes up, liquid water turns to steam and creates thrust. as piston goes down, pressure drops letting more liquid turn to steam, albeit sucking up heat to do it.

are we better off with the ideal gas laws pushing on the piston, or does the steam create enough thrust to cool the charge down to convert more energy to motion?

[drmiller100]

Originally Posted by Ken Fry

Another well-thought-out post. My only nit pick has mainly to do with the above -- which seems like a non sequitur: the size of the tank (electric or gas) has no direct effect on fuel efficiency. (In both cases, there is of course the indirect effect of weight... but that fits into a discussion of vehicular efficiency rather than one on powerplant efficiency.) But you presented a bunch of good, solid, points.

Thanks,
Ken

if we say our average commute is 30 miles, but specs of the car say we MUST be able to drive 250 miles on occasion, the empty space in a gasoline tank has little cost.

But the extra space in the batteries is rather expensive in terms of money and weight. And weight for sure costs MPG.

[dcb]

my ninja has a range of 250 miles without trying, and cost like $1000, doesn't take up much room in the garage either if I need to go that far Nobody is saying your local electric commuter MUST go 250 miles, or that you cannot hitch up a small ICE to the trailer hitch on the possibly rare occasions when it does need to.

[Diesel_Dave]

Originally Posted by drmiller100

interesting.
i did not know you could have higher intake pressures then exhaust pressures. thinking about it, theory says it sure is possible. though.

thank you.

You're welcome. One thing that's worth mentioning is that most EGR engines cannot have higher intake pressures--otherwise the exhaust wouldn't flow into the intake. That's one of the reasons some recent diesels (including my truck) are fitted with intake throttles. In situations where the intake pressure is too high to get enough EGR flow, the intake throttle is closed down to increase pumping losses. One more way EGR hurts fuel economy on a diesel.

On a gas engine EGR can actually help the pumping losses because more EGR allows the throttle to be more open (because fuelling is deterined by the oxygen content in a gasser), therefore EGR raises the intake manifold pressure.

[Ken Fry]

Originally Posted by drmiller100

But the extra space in the batteries is rather expensive in terms of money and weight. And weight for sure costs MPG.

Definitely.