Maximum fuel economy from a small 4-stroke engine.

[cheesehead]

I'm on a team which is building a small car to compete in a fuel economy competition. We're basically building a carbon fiber roller skate, and mounting to it a small (about 2 horsepower) gasoline engine, and seeing how much fuel it uses after a few 30mph laps of a racetrack.

What I'm wondering is if anyone has any suggestions for improving economy. Carburetor tuning may allow for optimized efficiency for a specific engine speed, but electronic fuel injection would be even better....if I could figure out how to add it to a ~60cc engine. Shaving the engine block to increase the compression ratio might increase efficiency....but what if I destroy the engine? And could I try the classic (but expensive) "port 'n polish"?

 

[lxskllr]

You could run the fuel mixture extremely lean. You'll burn up the engine in short order, but it might be enough to get you where you need to go.

 

[bobsmith1492]

Could you use a diesel engine? Could you turbo/supercharge it? Could you increase the compression and use super-high-octane gas?

 

[Comdrpopnfresh]

Turbo charge for sure. In thermodynamics I learned having a turbo charger reduces the amount of energy needed to just keep the engine going. That, and increase the compression ratio, and have more free-flowing throttling. But yeh- diesel is way better.

 

[Gibsons]

even if you can't turbocharge, the engine exhaust can be a source of energy. Maybe...

 

[CurseTheSky]

A lot of the things mentioned here increase horsepower, not fuel efficiency. Is it a race to see who's vehicle can go the fastest while using the least fuel, or who can complete the course with the least fuel used?

Things like superchargers, turbos, etc. will actually increase an engine's fuel consumption (extra fuel is usually used for cooling). The reason they're used on most fuel efficient cars is to bring the power to an "acceptable" level (I'd bet most people would choose a 170-200 HP, 27MPG car over a 120-140 HP 32 MPG car). Porting, polishing, intake, etc. will all help, but again, they're more focused on power than fuel consumption.

Assuming you just need to complete the course (and it's not a race), you should focus on things like gearing (if any; sacrifice acceleration for higher speeds / lower RPMs at a given speed), weight, and aerodynamics. Engine work will help, but those are what will make to break your design. For example, an aluminum engine block instead of cast iron, small wheels in the front, large in the back, etc.

If it IS a race, then you'll need to graph expected power versus expected increase in fuel consumption for each modification you want to do, and pick the things that have the best ratio. In that case, things like turbos should be a huge benefit, since the power gain should be much greater than the consumption.

 

[JimO]

If it is just about fuel efficiency and not about speed, you should look at stategy. As far as I know, most contestants at most-miles-per-fuel races don't run the engine constantly. They run the engine at the most effecient rev and stop the engine when they gathered enough speed. They keep running without engine untill speed drops too far and start the engine again. You need a lot of testing to find the best number of revs and best speeds to stop and start the engine.

 

[bobross419]

If you are using a roller skate then definitely consider talking to someone at a skateshop about the best wheel bearings possible. I know there is a huge difference in friction between the stock walmart cheapies and ABEC-3s (If they even still use the Abec System) - A world of difference. Also, try reducing the number of wheels to help alleviate even more rolling resistance. If you can get away with 2 wheels and still have a stable platform go for it, but you might have to stick with 3 depending on the size of your engine. At 30mph Aerodynamics probably aren't going to play a huge part, but streamline the design as much as possible. Reducing rotating and overall mass will help you out a great deal. I'd steer towards the EFI if possible, but stand alone programmable systems can be quite pricey - You could look into the MegaSquirt system, but some programming knowledge (C+ I believe) might be needed for the best possible tune. A well tuned carburator can pull in excellent number as well, especially if you are operating within a very small powerband.

Increasing low rpm torque should probably be focused on more than HP... the torque coming in lower on the RPM band will allow you to run a higher gear at a lower RPM. Increasing the compression ratio will help out with this, also advancing the cam timing a bit should help as well. If the rules allow then you could consider using water injection and high octane "race fuel" to allow you to bump up the compression even more. Another thing to look into would be Singh Grooves - They 'supposedly' allow you to run higher compression with less fear of detonation, but I haven't ever found any definitive tests one way or the other. Depending on how in depth you are getting on this you might want to try different exhaust/intake lengths/diameters to fine tune the engine for maximum performance at the lowest possible RPM.

If you can develop a launch system using some sort of stored energy to aid in the initial acceleration (I've got a mental image of a giant rubberband) then you can focus on maintaining a speed. Pulse and Glide methods like Jim0 was describing above have been extremely successful on automotive vehicles (both hybrid and non) often increase the MPG of a vehicle by double the constant speed MPG. Careful tuning of the power band and gearing should allow you to maintain an average speed of 30 mph by accelerating slowly to 35 then running with the engine off down to 25. Like Jim0 suggests, make sure you do a lot of testing to find the most efficient pattern for your particular engine and gearing.

 

[blahblah99]

Total weight, drag coefficient, power loss from engine to wheel, tire selection, and most importantly, tuning. Work on all of those if you want superb fuel economy.

 

[PlasmaBomb]

Originally posted by: Cheesehead
after a few 30mph laps of a racetrack.

Define a few? Like 5? If so long term reliability isn't too much of a worry (unless you intend to compete in multiple events...).

 

[Howard]

Increase compression as high as possible (may need to rework combustion chambers and piston crowns)
Coat piston skirts with friction-reducing material
Machine crankshaft counterweights to reduce drag
Minimize exhaust gas reversion... without doing a lot of trial and error, you could put in port extensions; see Figure E1 here - otherwise, you can play around with the exhaust header to maximize exhaust velocity while minimizing the loss of fresh mix (that you get once exhaust velocity increases beyond a certain point)

You might get some better responses in the Garage forum

 

[bobross419]

Also, try using only 2 piston rings instead of 3 like the Metro XFI.

 

[Pulsar]

Are you allowed to run a 2-stroke engine? Huge weight savings to be had there in the engine itself, not to mention you won't have quarts of oil doing nothing but lubricating.

 

[dkozloski]

A rule of thumb for many years has been to size the engine so that it will be operating at 80% of full rated power when doing its job. This gives a good balance between friction losses and pumping loses. As the engine increases in size the friction losses prevail, as the size goes down you have to wind it up to get the desidred power and pumping losses go up.

 

[KIAman]

Resistance
1. Minimize ground contact with tires
2. Wheel bearings
3. Gear resistance
4. Minimize drag co-efficient
5. Minimize frontal area
6. Polish internals of engine

Weight
1. Use lightweight materials for body construction
2. Use lightweight materials for engine (aluminum, or titanium if you can get exotic)
- Titanium is about 60% heavier than aluminum but 2x stronger so you can use 1/2 as much
3. Fill tires with Helium
4. Find a skinny driver

Engine/Transmission
1. Open air intake
2. Free flow exhaust
3. Direct fuel injection
4. Minimum amount of gearing needed transmission
5. High compression ratio
6. Diesel engine cycle (compression ignition)
7. Air cooling

 

[Howard]

Originally posted by: KIAman
Engine/Transmission
1. Open air intake
2. Free flow exhaust
3. Direct fuel injection
4. Minimum amount of gearing needed transmission
5. High compression ratio
6. Diesel engine cycle (compression ignition)
7. Air cooling

1. Please clarify
2. Please clarify
3. That's going to be a tough retrofit
6. Well, diesel engines run more efficiently, but you can't explode gas as of yet
7. Good call

 

[KIAman]

Originally posted by: Howard

Originally posted by: KIAman
Engine/Transmission
1. Open air intake
2. Free flow exhaust
3. Direct fuel injection
4. Minimum amount of gearing needed transmission
5. High compression ratio
6. Diesel engine cycle (compression ignition)
7. Air cooling

1. Please clarify
2. Please clarify
3. That's going to be a tough retrofit
6. Well, diesel engines run more efficiently, but you can't explode gas as of yet
7. Good call

1. No filtering or bends in the intake airways
2. No muffler or converter or bends in the exhaust
3. Yup
6. HCCI (Homogeneous Charge Compression Ignition) - bleeding edge tech

 

[Howard]

Originally posted by: KIAman

Originally posted by: Howard

Originally posted by: KIAman
Engine/Transmission
1. Open air intake
2. Free flow exhaust
3. Direct fuel injection
4. Minimum amount of gearing needed transmission
5. High compression ratio
6. Diesel engine cycle (compression ignition)
7. Air cooling

1. Please clarify
2. Please clarify
3. That's going to be a tough retrofit
6. Well, diesel engines run more efficiently, but you can't explode gas as of yet
7. Good call

1. No filtering or bends in the intake airways
2. No muffler or converter or bends in the exhaust
3. Yup
6. HCCI (Homogeneous Charge Compression Ignition) - bleeding edge tech

2. I don't know if this is actually going to increase fuel efficiency (the part about no bends)
6. I know. How is he going to explode gas in a regular engine when even researchers haven't perfected it yet?

 

[Pulsar]

2. Backpressure (pressure opposing the flow of the exhaust gas) has a direct effect on horsepower, and by extension, fuel economy. In fact, high-tech headers are designed with specific lengths to match the exit timing of the gases from each cylinder to reduce backpressure to a minimum.

 

[KIAman]

I also forgot to include intake resonance. If you can optimize the air pulses with the intake valve timings, you can effective get free supercharging.

 

[Howard]

Originally posted by: Pulsar
2. Backpressure (pressure opposing the flow of the exhaust gas) has a direct effect on horsepower, and by extension, fuel economy. In fact, high-tech headers are designed with specific lengths to match the exit timing of the gases from each cylinder to reduce backpressure to a minimum.

But what effect does this really have? You absolutely do not want to waste any fresh mix by pulling out too much, whereas it is safer to err on the side of some exhaust left behind. In fact, isn't that what EGR is about?

 

[bobross419]

Also, figure out you efficiency before going in and only use only the amount of fuel needed to reach the required distance. This will help you cut down on weight a little.

 

[fleabag]

Originally posted by: Cheesehead
I'm on a team which is building a small car to compete in a fuel economy competition. We're basically building a carbon fiber roller skate, and mounting to it a small (about 2 horsepower) gasoline engine, and seeing how much fuel it uses after a few 30mph laps of a racetrack.

What I'm wondering is if anyone has any suggestions for improving economy. Carburetor tuning may allow for optimized efficiency for a specific engine speed, but electronic fuel injection would be even better....if I could figure out how to add it to a ~60cc engine. Shaving the engine block to increase the compression ratio might increase efficiency....but what if I destroy the engine? And could I try the classic (but expensive) "port 'n polish"?

The key is fuel mixture ratios, valve timing and spark timing. Other things to consider are compression ratio and stroke (longer stroke being lower RPM but more torque).

 

[jhu]

are you allowed to coast with the engine off or do you have to maintain a set speed?

 

[PlasmaBomb]

I don't know if cheesehead is even reading this any more...