Engine and fuel efficiency. And other major factors.

[m.glisson003]

Obviously the smaller the engine the more fuel efficient it is compared to a larger engine. But how does one compare engines that have the same amount of cylinders to determine fuel efficiency between them?

Also, what are other major factors that affect fuel efficiency that I can influence?

[534BC]

I'm not sure what ? you are asking.

1 is size of engine (no mention of cyclinder number) and you have it corrrect.
2 If the number of cylinders is kept constant then the same is true, smaller is better.
3 If the size is kept constant then the lower cylinder number would be better.

mixture,compression,VE,other engine controls, ect. intake exh, head design. Most all of this is already done for us though.

[jdmccright]

A few factors come up off the top of my head.

1) Drag coefficient. The lower the drag the vehicle has going through the air, the less power that is lost pushing the car through the air. Drag increases as the square of speed, so it doesn't really consume significant power until the car is at highway speeds...this is why they had the old 55 mph speed limits. That was the "sweet spot" for fuel efficiency for vehicles back then.

2) Gear ratio. For max fuel efficiency, you want the car to be moving as fast as possible with the engine turning as slow as possible. This achieves the longest distance for the least fuel burned. Obviously there is a compromise here since no vehicle can make power when the engine's RPMs are at zero, and most gasoline engines have poor low-RPM power production characteristics. Diesel engines are better at low-RPM power.

3) Fuel type. Diesel engines are better at fuel economy for two reasons. One they can produce more power at lower RPMs than a similar sized gasoline engine because of the type of ignition used (high-compression versus spark). Two, diesel fuel contains more energy per unit volume than gasoline.

4) Driving habits. This is the most influential factor that you can change for free, and it is really a change in driving philosophy where you can't think that you have to be somewhere NOW. Think about it...why accelerate hard from a stop light if you can see ahead that you'll have to stop again at the next one? Keeping the revs low, accelerate slowly, let the car shift into higher grears sooner. Then let off the accelerator to coast (if you have an automatic transmission)...without using any gas at all...until you have to stop again. No sense in trying to "race" the guy next to you since you're all going the same direction. If he wants to get in front of you, let him waste gas to do it. Studies galore have shown that an aggressive driver will arrive at their destination only about 30 seconds before one that is more conscious. Yes, it's a boring way to drive, but it WORKS to save fuel. I could expound much further but I hope you get the idea.