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Gas Mileage...


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Ok now I am starting to get really curious about what can make a gas engine more efficient... I would seem to me that weight of course is a factor, But doesn't making an engine more efficient, also involve modifying it to in essence to produce more power, but then restrict the amount of air / fuel available. Meaning keeping the air fuel mixture just on the lean side and also not allowing max acceleration, so if using a carb, setting up a single barrel on it?

 

Am I missing things? Would that make much higher mileage?

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takes a certain amount of power to do a certain amount of work. How efficient the fuel is used determines the economy. Along with friction, pumping losses, weight and aerodynamics. Maximize all of those things and you maximize economy.

 

Friction and pumping losses can be reduced in a smaller displacement fewer moving parts engine (ie 4 banger) Throw that in a lightweight, aerodyamic vehicle and get killer mileage.

 

Note that you have to optimize efficiency in the range you plan to run the engine. In these tiny gas motor/electric hybrids, some of them have engines that operate at a fixed rpm, one of maximum efficiency. (Gasoline engines can be tuned to produce volumetric efficiency better than 100% in small windows of rpm)

 

Gasoline engines are most efficient at WOT (because of pumping losses and throttling) which also seems counterproductive to good fuel mileage. So hammer down (stop spinnin' those tires) and get up to the speed, then keep it as constant as you can for as long as possible.

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I just gear out to avoid pumping losses. Shift by 1,000 in the truck (almost if not flooring it, 1st, 2nd, 3rd). Then once I am on the boulevard I can sit in 5th at 1,000rpm and just give it the tiniest of touches to keep it rolling. Helps a lot for city mileage. 70mph on the freeway though, not much you do but get a less powerful motor so the TB is open more and there is less friction from smaller displacement.

 

It has worked for me in my instance, but probably won't for everyone depending on their EFI or Carb. Also when you go to WOT emissions stuff typically gets disabled. Maybe right before WOT is best unless you program the computer differently so that your not clogging cats or over fueling.

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What would the best way to build a very high fuel mileage engine?

 

Build a diesel! :) Since diesel fuel has more useable energy per liter than gasoline, it makes sense to use other fuels that will provide more energy for a given mass and weight such as diesel.

 

I think generally speaking, engines with more stroke than bore are more efficient, though I am just guessing on that one.

 

Definitly the less cylinders the more efficient, but to a point.

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Ok... that's driving it... what about building an engine specifically for fuel mileage. Would a high compression engine more efficiently use the available fuel air provided? What would the best way to build a very high fuel mileage engine?

 

Well. I would shoot for a small engine that makes JUST enough horsepower at WOT for cruising. Then I would give it a relatively high CR with a relatively small turbo, good EFI tuning... Thats about the most I would hope for from a gas motor.

 

If you take a diesel for example, its not that the fuel is THAT much more powerful its more that there is no need for a throttle body which the engine fights against, they are often turbo which reuses some exhaust energy and such, and they have a compression ratio through the roof.

 

The limit is going to come when you need power for acceleration and such, which on a gas motor inevitably means that your cruising is going to be with the TB 80% shut or so, which is a large restriction. The choice then is, do I want a motor that just barely gives enough power to maintain 65mph (~20hp) or do I want one that can scoot. Which is why fuel economy is an issue. A 20hp motor might be very compact though and worth installing with some sort of one way engagement.

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