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Question 2 Three cylinder or four cylinder engine?

Car engine operates on 4 Stroke cycle and crank rotates 720 degrees (two revolutions) in one cycle. Strokes are:

  1. Suction stroke (0 to 180 degrees)

  2. Compression stroke (181 to 360 degrees)

  3. Power/Combustion stroke (361 to 540 degrees)

  4. Expansion stroke (541 to 720 degrees).

Only power stroke produces energy via burning the fuel. There are two major factors which need to be consider in selecting a car based on this criteria; Power and Friction losses.

Power in four cylinder engine

Four cylinder will have 4 power strokes and they have to be adjusted in overall engine cylcle of 720 degrees. This means firing should happen at interval of 180 degrees each (720/4=180). Figure shows power stroke and their firing duration for 4 cylinder engine.

Power in 4 cylinder engine

Power distribution in 4 cylinder engine

It can be observed from figure that there is no gap between cylinder firing and power production remains intact. You will feel smooth functioning even at lower rpms.

Power in f three cylinder engine

It will have 3 power strokes, each for each cylinder. This requires firing should happen at 240 degrees interval (720/3=240). But, strokes takes 180 dergrees to complete and where the rest of 180 degrees (720-540=180) gone? There is no firing in this interval and engine runs on its own inertia. It can be seen from following figure:

Power in 3 cylinder engine

Power distribution in 3 cylinder engine

It can be observed from figure that for the duration of 60 degrees each there is no power production. Actually, instead of putting idle 180 degrees at one place it is divided at 60 degrees of interval and equally distributed. This will give less fuel consumption.

Friction losses

Consider the volume is same for both the cases. If we are talking about friction then we must take in mind contact area between piston and cylinder.

Volume of 3 cylinder engine=Volume of 4 cylinder engine

3*pi*d32 l/4=4*pi*d42*l/4

d3=1.15*d4

Frictional surface area of engine is

Surface area of 3 cylinder engine = 3*pi*d3*l

= 3*pi*1.15*d4*l

Surface area of 4 cylinder engine = 4*pi*d4*l

As4/As3=4*pi*d4*l/3*pi*1.15*d4*l=1.16/1

This means surface area of four cylinder engine is 16% more than three cylinder engine. More surface area more friction and more fuel usage.

Summary on question

Following important observations can be drawn

  1. Four cylinder engine gives continuous power supply while three cylinder engine doesn’t.

  2. Theoretically speaking, three cylinder engine has 16% lesser friction losses.

If you are running your vehicle mostly on highways then you won’t feel three cylinder engine is underpowered because of sufficient inertia of the vehicle. You will feel three cylinder engine is underpowered when running in city traffic because you will run vehicle mostly at lower rpms. In this case four cylinder engine will give better functioning in city traffics. Also, three cylinder engine has lesser frictional loss. So if you are running your vehicle mostly in cities then better go with four cylinder engine.

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