Choosing the Right Engine

There are different types of engines: two-stroke piston engines, four-stroke piston engines, and the Wankel rotary engine.

Two-stroke engines 

This is an internal combustion engine where air and fuel burn inside the cylinder. This is the engine most commonly used in lawnmowers and small two-wheeled vehicles. This piston engine operates on a two-stroke cycle with linear movements.

This type of engine wears out faster than others.

Four-stroke engines

This is also an internal combustion engine. It’s the type of engine used in the automotive and aerospace industries. Four-stroke engines operate similarly to two-stroke engines, except they follow a four-phase cycle. These four phases are:

• Intake: the air-fuel mixture enters the cylinder
• Compression: this involves compressing the gases in the combustion chamber
• Combustion: compressed gases ignite and combust
• Exhaust: gases are pushed towards the exhaust to be evacuated

The Wankel engine 

This internal combustion engine is a rotary piston engine that differs from two-stroke and four-stroke piston engines in that it generates no reciprocating motion. It therefore makes less noise and generates less vibration than piston engines. This engine is sometimes used in cars, notably the Mazda RX-8. Today, this type of engine is being considered for hybrid engines.

Engines have different configurations.

The straight engine

Also known as an inline engine, this engine is the most commonly used, the easiest to install, and the least expensive. In this engine, the cylinders are arranged in a single row, giving it an elongated shape. The disadvantage of this configuration is that it requires more space lengthwise. Moreover, this inline configuration unbalances the motor and can lead to vibrations. However, it is possible to add a counterweight to attenuate these problems.

The flat engine 

This completely flat engine features horizontal pistons. Unlike the inline engine, the cylinders of the flat engine are located on either side of the crankshaft to ensure better engine balance.

Another advantage of this flat configuration is that it takes up little space in terms of height, allowing for very low positioning of the engine on the chassis, which gives the vehicle better roadholding.

On the other hand, flat engines require maintenance and the replacement of spark plugs.

The V engine

As the name suggests, the V engine is V-shaped, with two rows of cylinders located above the crankshaft that operate alternately (in a V8, there are two rows of four cylinders). 

The V engine has several advantages:

• The type of crankshaft used in this type of engine is very light, which reduces vibrations in the vehicle.
• It offers high starting torque even at low engine speeds.

However, the complex configuration of this engine makes it very expensive.

The W engine

This engine works similarly to the V engine, but its configuration is doubled, hence the W. The cylinders of the W engine are staggered. Its main advantage is that it takes up very little space.

This type of engine is used in aircraft and also in high-end model cars. Like V engines, W engines are very expensive.

When choosing an engine, you need to determine the maximum power you require (expressed in horsepower [cv] or kilowatts [kW]), which is defined by the engine torque (expressed in Newton meters [Nm]) and the engine speed (expressed in revolutions per minute [rpm]).
Since engine torque varies according to speed, you need to check that the engine you’re interested in is suitable for your application. For example, if you choose to invest in a gasoline engine, it will need to run at relatively high speeds to reach its full torque potential. On the other hand, if you choose a diesel engine, it will be able to run at low speeds and reach high torque while limiting fuel consumption.

As a general rule, manufacturers supply charts showing engine power and torque as a function of speed. This information will help you decide if the engine in question is suitable for your application.

Gas engines are derived from gasoline or diesel engines. The power delivered by these engines is therefore equivalent. You should choose this type of engine if your primary concern is being more environmentally friendly.

4-cylinder engine

The total engine size corresponds to the volume of each cylinder multiplied by the number of cylinders. As a general rule, for engines with the same total engine size, the one with more cylinders will be more efficient and has greater power. Multi-cylinder engines are therefore more efficient than single-cylinder engines.

Engine performance also depends on the number of cylinders. For example, a single-cylinder engine has little torque at low rotational speed and full torque at medium rotational speed. Increasing the number of cylinders generally increases engine torque at low and high rotational speeds.

The number of cylinders also has an impact on operating comfort. In general, the greater the number of cylinders, the lower the vibrations generated by the engine. On the other hand, the size of the engine is increased.

Four-cylinder engines used to be commonplace in small engines, but in recent years they have been increasingly replaced by three-cylinder engines, which are more economical both to buy and to run, and more efficient in terms of fuel consumption and pollutant emissions.

These engines must be artificially cooled, either by air or water. An air-cooled engine is lighter and takes up less space than a water-cooled engine, but engine temperature management is more complex. You need to take this into account because the efficiency of an engine is directly related to its operating temperature.

Gasoline-powered engines are generally 4-stroke engines, but there are also 2-stroke engines whose fuel is a mixture of gasoline and oil. These engines are simpler in design, but their rotational speed must be higher than that of an equivalent 4-stroke engine to obtain the same power. What’s more, 2-stroke engines are tending to disappear, as it is very difficult for manufacturers to offer 2-stroke engines that comply with anti-pollution standards.