3-cylinder engine vs. 4-cylinder engine: pros and cons

An increasing number of petrol cars are using three cylinder engines these days. However, there’s a perception that three-cylinder engines are inferior to four-cylinder engines. CarToq takes a look at this common myth.

There are a number of cars in the Indian market with three-cylinder petrol engines. The Maruti Alto K10, Maruti Wagon-R, A-Star and Estilo all sport three-cylinder 1-litre engines. The Maruti Alto and Hyundai Eon sport three-cylinder 800 cc engines. The Volkswagen Polo petrol and diesel and Skoda Fabia petrol and diesel have three-cylinder 1.2 litre engines, while the Polo GT TSI has a 1.2 litre four-cylinder turbo-charged petrol engine.

The Chevrolet Beat diesel has a three-cylinder engine that’s highly fuel efficient! The old Hyundai Accent Viva CRDi had a three-cylinder 1.5 litre diesel engine. The Ford EcoSport EcoBoost petrol SUV too has a turbocharged three-cylinder petrol engine, but with power output of 123 bhp, which is equal to what a 1.6 litre petrol would produce!

So why are three-cylinder engines becoming popular?

engine-comparison-photo

Advantages

Fuel Efficiency

The basic advantage of a three-cylinder engine over a four-cylinder is that it is inherently more fuel efficient (as there’s one cylinder less of volume of fuel to burn). The smaller the engine size the less fuel it will burn and hence it is more fuel efficient. Of course, if a four-cylinder and a three-cylinder engine have the same capacity (for example 1000 cc), then theoretically they should use the same amount of fuel. But still the three cylinder will be more fuel efficient. Here’s why.

Lower frictional losses

Since there is one cylinder less in a three-cylinder engine vs a four-cylinder engine, there is a lower surface area of metal-to-metal contact (pistons moving inside the cylinder) compared to a four-cylinder engine and fewer joints (piston to crankshaft). This makes for better mechanical advantage as less fuel is wasted in overcoming friction.

Lighter weight

The obvious advantage of having a cylinder less is that there is considerable weight saving in the engine. This gives carmakers a lighter engine, which translates to lower kerb weight for the car and hence marginally better fuel efficiency too.

Compact size

Another advantage of having fewer cylinders is that it allows carmakers to account for tighter packaging in the car. The engine won’t take up too much space in the engine bay, allowing carmakers to design cars that have maximum cabin space and minimum space for the engine.

Cost saving

With the rising cost of raw materials (steel and aluminium), it is cheaper for a carmaker to build a three-cylinder engine than a four-cylinder one. This will also translate to some cost savings for you the buyer as well, if the carmaker chooses to pass on the advantage.

However, there are some inherent disadvantages of a three-cylinder engine over a four-cylinder.

Disadvantages

Not refined

A four-cylinder engine would be much smoother sounding than a three cylinder engine. That’s because a normal internal combustion engine is a four stroke cycle – intake, compression, combustion and exhaust (suck, squeeze, bang, blow). So at any point in time in a four-cylinder engine, there is one cylinder that is always on the power stroke (combustion) in the cycle. In a three-cylinder engine there is a difference in the way the pistons are arranged on the crankshaft, which leads to a delay of half a cycle between power strokes. In a three-cylinder engine a power stroke happens after every 120 degree rotation of the crankshaft, while in a four-cylinder engine it happens with every 90 degree rotation of the crankshaft (for one full 360 degree rotation of the flywheel). This shows up as a slight gap in the firing time, and hence a rougher engine note.

Needs more revving

During the time that no power stroke or firing is taking place in the engine, the engine is moving only on momentum generated by the flywheel connected to the crankshaft. At low speeds, this can lead to more vibrations and if not enough throttle input is given, it can also stall. But at higher revolutions per minute (RPM), there is much better balance in the engine and it is much smoother. The flywheel gains better momentum. For better power, a three-cylinder will need to be revved to higher rpms than a four-cylinder of equal capacity.

In terms of power capacity, three cylinder engines these days can be made to generate as much or more power than an equivalent four cylinder engine, using other associated technologies. For instance, adding four valves per cylinder (two intake valves, and two exhaust valves), will allow an engine to breathe better and therefore respond faster. Similarly, turbocharging an engine, will allow for higher compression and more power generation from the engine. Also read: How to tune your petrol car for more performance

If you want more refinement and a quieter car, with a bit more power  for equal engine capacity, then a four-cylinder engine is better than a three-cylinder. But if fuel-efficiency and lower purchase cost are your priorities, pick a three-cylinder engine. Also see: Performance modifications for your diesel car

Share your thoughts and experience with three- and four-cylinder engines with the CarToq community.