3 vs 4 cylinder

[guitarterry]

Can anybody tell me why a 3 banger would get better gas milage than a 4 banger at a lower rpm. Once you slow the 4 down where the frictional losses are the same for the motor, then the alternator, waterpump, etc would be turning slower, thus more mpg, plus the mechanical advantage- extra spark plug, more flow, etc. of course you would want to recam to better specs.

[BlackDeuceCoupe]

Originally Posted by guitarterry

Can anybody tell me why a 3 banger would get better gas milage than a 4 banger at a lower rpm.

What's your theory?

[Yaristock]

Hmmm, this almost....nvm; its better because you have one less injector shooting out fuel, thus you are using less. The advantages you stated above are the same for the 3banger.

[guitarterry]

Ill answer both of the above question here. theory- 3 banger at 3200rpm=a 4 banger at 2400rpm on terms or power, frictional loss and yes even injector firing (3 injectors firing at 3200rpm=4 injectors firing 2400 rpm) Now that the injectors are injecting the same, like a underdrive pully, everything else is turning slower (alt,water pump, etc)

[Yaristock]

You are technically correct that the four banger would be more efficient if we were to compare power ratios, but assuming that they both idle at the same rpm we wouldn't factor that in; because at idle(no accessories) what does that extra piston do other than create heat, and unused power?

edit: what you did above would be the same as saying a v8 is just as efficient as a i4 because at 2800rpm the v8 makes this much power when the i4 must be at 5000rpm to make the equivalent power.

[guitarterry]

first the 3 banger idles higher than the 4. second you are comparing apples to oranges. the auto companys are taking the v8's and making v10's so that they make the same power with more mpg. the reason they dont make 1300 v8s with the same tricks of the xfi is cause of the cost. if we use ur reasoning the yamaha vstar (1300 v twin) gets 42, the 98 metro 3 banger gets 44. the ford festiva gets 44.4 and the honda cb 750 gets 45. so as the number of cylinders increase the milage goes up in that example.

[dcb]

This is a little hard to explain, and I have to make some generalizations...

If you hold displacement and stroke equal, an engine with less cylinders will be more fuel efficient. The ideal (and the limit) being one cylinder, where the shape of the total piston area is a single circle.

Lets say that you have a 2 cylinder engine with pistons of radius 1, the total circumference around both pistons is 12.57.

Whereas if you have a single piston engine with the same stroke/displacement it would have a piston radius of 1.414 and a total circumference of 8.88.

This means that the single piston engine has about 1/3 less contact with the cylinder wall, and thus significantly less friction, even though the stroke and displacement are identical.


Also consider that less pistons means less connecting rods and bearings, and less valvetrain and camshaft lobes.


And if we are comparing a larger 4 cylinder to a smaller 3 cylinder, the larger 4 cylinder is going to have more pumping losses for a given power output

[guitarterry]

metro 1.0 91
45 city
50 hwy
metro xfi 91
53 city
58 hwy
metro 1.3 97 metro1.0 97
39 city 44 city
43 hwy 49 hwy
projected 4cyl xfi 97
47 city
51 hwy

advantages of a 4 banger xfi. lower rpms cruizing, more power than the 3, no worries about trashing the second cylinder when the egr valve fails. even more dependability since u are using less rpms and more cylinders.

[guitarterry]

ok volumetric efficiency and parasitic losses would be better with a 4, but Engine efficiency improves at higher rpms. thats why u go small and up the rpms. on the flip side the frictional losses of the internals increase as the square of speed.

[Bicycle Bob]

In automotive applications, it is better to use fewer, larger cylinders than racing tradition encourages. The peak RPM is reduced, but at cruising power that is irrelevant, and there is a greater volume to surface ratio in the cylinders, and hence a smaller percentage of the charge has its flame quenched by the metal.

The diesels that get 50% efficiency in ships have cylinders bigger than some rooms, and run at 110 rpm.