What are the advantages and disadvantages of the three, four, five and six-cylinder engines?
Since the birth of motorcars, there have been many successful and unsuccessful attempts to develop and universalize different types of engines.
Currently, manufacturers make extensive use of three and four-cylinder engines for regular cars and reserve the six-cylinder engines for large saloons and SUVs, as well as sports cars.
However, not everybody likes to go for the conventional engine choice, as Audi shows with their latest RS3. Their stubbornness to not dispose of the five-cylinder engine must have sound reasoning behind it, so we're here to take a look at all the engines mentioned above and understand what their advantages and disadvantages are.
Initially, this type of engine was developed for compact or small cars with a displacement of less than 1000cc. These days, the displacement of three-cylinder engines goes all the way up to 2000cc, with the most powerful one pushing out 600hp (Koenigsegg Gemera). Its small and compact design makes it easy for engineers to package a small engine to fit in a car, as it only requires a small amount of space to install.
The small size also makes it extremely fuel-efficient. However, the disadvantage of this engine is that the stroke is not large enough, so the torque that it produces is low, therefore, engineers use various innovations to bump up the torque and power, such as using VVT-i system, fuel injection, DOHC, and even turbocharging to make this engine more powerful.
There's also the problem of the unharmonious movement of the piston. In a three-cylinder engine, there are two pistons moving up and one piston moving down, causing the engine to vibrate while operating. The vibrations produced will resonate through the car and into the cabin, accelerating driver fatigue in long-distance driving. This vibration will also cause the engine mounting system to wear out quickly, and replacement costs are usually more expensive than 4-cylinder engines.
Some new engines, such as the ones from Volvo, have a balancing mechanism to control the vibrations produced by the disharmonious movement of the pistons.
The four-cylinder engine is a popular choice amongst manufacturers for many different reasons. It comes with a displacement of 1000cc all the way up to 2700cc for petrol engines and can go up to 4000cc for diesel engines used by trucks. This engine is preferred because it has a harmonious piston movement during operation apart from being compact and straightforward.
Two pistons go up, and at the same time, two pistons go down, canceling out any sort of primary balance vibration. Because it has fewer components, it is also lightweight and gives a dynamic advantage to the car. The engine is also cheap to produce.
But the disadvantage of this engine setup is the existence of secondary balance inertia on the crankshaft, which will cause some vibration. To overcome this problem, a balancing shaft must be fitted to the engine. Due to this problem, it is very rare to have this engine in a very large displacement.
Just adding two more pistons to a four-cylinder inline engine provides you a six-cylinder engine. Toyota, BMW and Nissan are very fond of the six-cylinder engine, especially for their sports cars. Power will come in at every 120 degrees in engine timing compared to 180 degrees in a four-cylinder, giving it a very smooth power delivery.
Just like the four-cylinder inline engine, this engine is simple in design, easy to build, and the construction cost is cheap. However, due to the large size of the engine, it isn't easy to arrange this engine in a horizontal orientation, so it is not seen on a front-wheel-drive system.
Usually, these engines are installed for rear-wheel-drive cars where the engines will be stacked vertically. BMW uses the six-cylinder inline engine in the Z4 and also the Supra but with a closed deck design.
So we've come to the odd engine out, the five-cylinder engine. There are many reasons why a five-cylinder is used as a powertrain. Chief amongst them is its shorter in length than an in-line six, which means it can be mounted transversely much more easily. This opens up the opportunity for the engine to be mounted in smaller cars within a manufacturer's range.
Some of the most excellent compact fast cars, such as the Audi Ur Quattro, second-generation Ford Focus RS, and the Volvo 850R, came with a five-cylinder engine. Delivery of power is smoother than that of a four-cylinder, and when enhanced with the vertical balance of the reciprocating pistons as well as its ability to produce a more significant displacement, there are plenty of reasons why the likes of Audi and Volvo love the five-cylinder engine.
The characterful engine sound it makes, makes it sound like a V10 - another reason why these two manufacturers love them.
Unfortunately, there is also a torque imbalance along the horizontal plane of the engine. What this means is that a five-cylinder engine is constantly trying to twist or flip over its length. This is usually solved with a balancing shaft but the balancing of the shaft is an art that only some manufacturers can master. They only master it because they need to.
The five-cylinders also exude more carbon dioxide than a four-cylinder, but manufacturers work around it by including a mild hybrid system, cylinder deactivation tech and a belt-driven starter/generator.