Maximum fuel economy from a small 4-stroke engine.

[NeoPTLD]

Burn fuel leaner than stoichiometric and run it hot. Tuning for maximum efficiency increases NOx emissions, so production cars are made with emissions control at the expense of fuel economy.

 

[bobross419]

OP, did you guys build your car already? What methods did you guys decide to go with?

 

[potato28]

I don't know a lot about small engines, but I know for sure on larger engines you need a certain amount of intake and exhaust manifold/ system or else the engine won't run efficiently. Running the engine extremely lean works in theory, but only up to a certain point. You would need an extra engine or 2 to test the points at which the engine would seize from the fuel air mixture to maximize the F/A lean mixture. EFI would also be very good for this as you could program the EFI for timings. Speaking of timings, what does this have for a valvetrain? OHV's make for a smaller and lighter engine, but more power is lost because of the lifters. SOHC would be something to look at, maybe even with VVT if you could find something like that on a small engine.

The ignition system would have to be computer controlled, if you had an ignition system at all. The only problem with all of the computer parts is they need to be protected from the elements, which requires some sort of an enclosure which would mess with the aerodynamics of the vehicle unless there's a pre-built engine compartment with a small area for these systems. If this was a larger engine (like a small 4 cyl. Honda 1.2L), I could help out more, but this is all I can think of for a small unknown sized or manufactured engine.

Or an even more radical idea would be abandoning a traditional Otto cycle piston engine and looking at a Wankel engine. For a similar size they do output much more power, and I think you would be able to find a small enough one to fit the need of this vehicle. The only need for it is a real EFI and ignition system, but I think it would make a better power source. If you want more info I suggest making a new thread in The Garage, as they have more experience with all types and sizes of engines in total.

 

[Howard]

Rotaries use more fuel than Ottos for a given amount of power.

 

[cheesehead]

Originally posted by: bobross419
OP, did you guys build your car already? What methods did you guys decide to go with?

Nope.

Honestly, it's still in the "We can't get everyone to show up at the same time and place to make a decision" stage of development.

It looks like I'm going to be trying a lot of old-school tuning tricks and spending some time on a dyno at the local moped shop.

Thanks for all the new ideas, everyone - hopefully I'll be able to use some of them.

 

[FishTankX]

I wonder if you could add a rankine bottoming cycle..

The exhaust powers a heat exchanger for a small resivour of water, generates steam which drives a steam turbine...

 

[Skud]

What about gas/diesel electric?

If you're going for efficiency then diesel is the fuel of choice.

Diesel --> ~130,500 BTU/Gallon
Gasoline --> ~115,000 BTU/Gallon

If there is no restriction on fuel (onboard energy) you could even add a fully charged battery to the system.

Engine --> Generator --> Battery --> Electric Motor..

This would allow you to run your gas/diesel engine at its peak efficiency RPM and with your electric motor you get 100% torque at all RPM.

Riley