balper said:
There is a discussion going on in my neck of the woods regarding the efficiency of engine with 3 cylinders. Is it true that 4 cylinder engines perform better, are easier to maintain, etc... or is it just an "urban legend"?
My 60 HP has a 3 cylinder PERKINS engine, and I feel kind of strange when people start to talk about the "altogether superior" 4 cylinder engines
Welcome to the TBN community!
First off, you do know the difference between a 3cyl and a 4 cyl, right?
One has an additional cylinder.
That's about it.
Now for the subtle details. Let's make an assumption that there are 2 engines that each have 200 cubic inches of displacement (3.3 liter). They will both have about the same Hp. Let's give them the same diameter cylinders. The 3 cylinder will have a longer stroke. If the cylinder diameter is 4 inches, the strokes will be 4 inches for the 4 cyl and 5.3 inches for the 3 cylinder. Operational effeciency and low speed torque are typically greater with a longer stroke as there is more time at the top of the stroke with the combustion pressure the highest. More chemical energy is turned into mechanical energy. This means more torque to the wheels for the 3 cylinder across all RPM's. Score one for the 3 cyl!
The 3 cylinder will also have less friction. It has 1 fewer set of piston rings to drag up and down the cylinder, 1 fewer set of wrist pin, rod and main bearings. It also has 1 fewer set of lifters, pushrods, cam lobes, rocker arms and valves to operate. Less friction is less wasted energy and less fuel used for the same power output. Score one more for the 3 cylinder. As the failure rate of engine parts is pretty constant (a rocker arm will fail every 30 billion actuations for example) across all engine types, fewer parts means fewer parts that can fail. That means longer times between failures.
If the engines have the same size oil pumps, the 3 cylinder will have more oil flow on each bearing surface (because it has fewer bearings to share the oil). That should make the friction surfaces run cooler and wear at a lower rate making the 3 cylinder even more reliable.
So, if you want an engine that has more torque to pull a plow, uses less fuel and is more reliable --> get the 3 cylinder.
So why are there 4 cylinder engines? You can get more displacement from more cylinders and that makes more power. At some point, the benefits of 3 vs 4 (or more) change where the stroke gets too long to be practical or the rotating parts get to heavy etc. About the biggest 3 cyl's made are in the 200 cu inch range (3.3 L)
Hope this helps you with your 4 cylinder buddies! Personally, I think they are just envious of your long stroke.
jb