Engine Efficiency

[Prime-Evil]

Here is a practical question. Car engines tend to be grossly overpowered. If you are willing to give up maximum speed and accelleration for fuel economy, what modifications could be made to an internal combustion engine, besides replacing it with a smaller one? Let's assume you want to cut the maximum horsepower down to 60hp or so, and increase the efficiency mostly on the lower end, like in the 10hp range. The intention would be to use the car mostly in town and on back roads, and try not to drive much over 80km/hr, except downhills of course.

[BhavinB]

Manufacturers have charts of specific efficiency versus RPM for all engines they make. To do what you say, we'd need to take the RPM that has peak efficiency (and has atleast say 60 hp), then design a CVT (Continuously variable transmission) such that it keeps that RPM for the majority of speeds up to 80 kph. Unfortunately, there are many engineering hurdles to doing this...mostly in the design of a capable CVT.

[Prime-Evil]

I would be happy just to get better efficiency when driving 40mph or 60kph.

How could I do that?

[YT2095]

change your gear box ratios then.

[BhavinB]

change your gear box ratios then.

 

You could use taller ratios, but realistically people just need to change their driving style to get a noticeable improvement in MPG. Don't ramp up from stop very fast, if you have manual try coasting to a stop (with the clutch down), use higher gears for cruising, etc. I used to get 25 MPG on average from a 1995 V6 Ford Mustang. In those days, 5th gear was my best friend.

[YT2095]

actualy the only REAL answer is to use a lower CC engine, it`s not like you could crimp/block the fuel-air mix to 2 heads and pull the plugs out and run on 2 cylinders instead

well, not for Long anyway

[BhavinB]

lol...getting a new car with a smaller engine is more expensive than changing your driving style. But I totally agree on the small engine concept.

[Prime-Evil]

That's sort of what I was getting at. Wondering if there was a way to make an engine effectively smaller. Maybe recirulating some exhaust to run leaner or converting to hydrogen fuel or running a turbocharger to depressurize the engine below atmospheric pressure.

 

It would be nice if you could buy engines off the shelf. Like take the 60hp desiel engine from a smart car and put it in a 4 seater, or even a minivan for just driving around town.

[augment]

check this out:

http://pesn.com/2006/05/02/9500266_Gun_Engine/

[Prime-Evil]

92% isn't possible, thermodynamically speaking. 40% would be very good. 20% would be a considerable improvement good if you could get it over a wide power range. The focus should be on being efficient at 5-10HP while still being capable of 60-80HP.

[H2SO4]

could you just strip the car of everything, including engine and transmission, and retrofit an electric engine, a belt drive, and about 8 deep cell car batteriers, and maybe a few solar panels on top to charge when your parked? There's a way not to buy gas at all. I theory this could work for around town commuting under 40 miles, if that.

[Prime-Evil]

The trouble with electric is to get good charging and discharging efficiency you need lots of batteries, and batteries add weight which means you need more power for the same performance, which means you need more batteries.

 

It's a trade-off. You end up 90% x 90% x 60% at best.

That is for AC/DC + Charging + Discharging.

You can add about another 75% because of wieght of batteries.

Also it usually takes fossil fuels to make electricity, so another 33%.

 

90% x 90% x 60% x 75% x 33% ~12%

So you can do just as well, or better, with a 10-20 hp diesel.

But electric is nice and quiet for short distances.

Also, the solar panels avoid a lot of innefficiency.

Again, keep the weight down, and the HP down.

Scooters are very efficient for one passenger.

At some point a bicycle is even better.

At some point, walking is best.

.

[H2SO4]

bicycles are incredible

[chilehed]

If by "besides replacing it with a smaller one" you mean that you want to keep the same basic engine and just do stuff too it, then you've boxed yourself into a bad position. The engine's going to displace the same volume, and the engineering compromises won't let you get both reduced power and increased thermal efficiency, especially if you're interested in things like drivability and the like.

 

If reduced displacement is indeed on the table, then I'd head in the direction of a small displacement turbocharged diesel cycle. You'll have lower pumping losses and motoring torque, higher thermal efficiency due to the compression ratio increase, and you'll be getting useful work from the otherwise wasted exhaust heat. Beyond that, just do all of the details well: piston and bore geometry, valve timing, fuel and spark calibration, lube system pressure and viscosity, materials selection, etc, etc.