The internal combustion engine is one of the most important sources of power for cars. Through the combustion of fuel, power is generated here, which is available to drive the vehicle. In this guide, you will gain extensive insight into the technology of the combustion engine.

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The displacement of the internal combustion engine

The displacement indicates the size of the volume that results from the individual cylinders of the combustion engine. This volume is calculated from the working stroke of the piston stroke and the cross-sectional area of the piston. In the case of internal combustion engines, the displacement refers to the volume displaced by the stroke of all the pistons together. For example, an engine with a displacement of two liters or 2000 cc may have four cylinders, each with a volume of 500 cc.For engines in cars, the displacement is an important characteristic value. Here, the displacement indicates the amount of ignitable mixture that can be used to achieve power with each piston stroke.

The relationship between displacement and power output

The displacement of an engine affects its overall performance. Often, greater engine displacement is accompanied by greater power output. A car with a larger engine often offers more engine power than the same model with a smaller engine of the same design. However, more displacement in liters or cubic centimeters does not necessarily mean more power in all cases. After all, performance, measured in kilowatts or horsepower, also depends on engine technology and many other factors. If the engine’s displacement is unnecessarily large, fuel consumption and pollutant emissions can increase. If the engine displacement is too small, on the other hand, increased engine speed may be necessary to achieve the power required in each case.

Power and torque at different engine speeds

The power of an internal combustion engine is determined by torque and speed. The torque indicates the force acting on the pivot point of an axle via a lever. The value of the torque is given in newton meters. In the case of an internal combustion engine in a car, a high torque should be applied over as wide a speed range as possible. However, the relationship between power, speed and torque can vary depending on the engine and engine design.

The relationship between speed and torque

When an engine reaches its maximum torque depends in each case on the design and type of engine. In the case of internal combustion engines, maximum torque is often already available at lower engine speeds than the maximum possible output.Modern diesel engines with turbochargers already reach their maximum torque at low engine speeds. These engines allow the car to accelerate effortlessly even from a low engine speed range. Gasoline engines without turbocharging in particular, on the other hand, require a certain engine speed to achieve high torque. Here, maximum torque is often only available in the upper speed range. Downshifting is then necessary to accelerate powerfully with the car.

Power-to-weight ratio and acceleration

The power-to-weight ratio describes the relationship between the vehicle mass and the engine power. This figure can provide an indication of the vehicle’s acceleration capability. The power rating of the engine alone says nothing about the driving lines that can be achieved with a car. For example, a vehicle may be equipped with a powerful engine but have a high vehicle weight. In this case, there could be a poor power-to-weight ratio. The vehicle in question has a lot of power, but its high weight means it takes a long time to accelerate. By contrast, a particularly lightweight vehicle, such as a sports car, achieves a better power-to-weight ratio with the same high engine output. Today’s automakers are striving for a low power-to-weight ratio with a balanced relationship between vehicle weight and engine output.

Acceleration as a function of power

The acceleration time of cars is generally specified from 0 to 100 km/h with a value in seconds. For example, sports cars accelerate from a standstill to a speed of 100 kilometers per hour in four seconds, whereas a normal passenger car may take ten seconds or more to reach 100 km/h. Often, cars with higher engine power accelerate faster than low-performance vehicles. When accelerating from a standstill, the torque of the engine is more important than the pure engine power. The power in turn is decisive for achieving a high maximum speed. In addition, the acceleration behavior of the vehicle depends on other factors, such as the installed transmission.

Different engine designs in automotive engineering

Different engine designs are used in automotive engineering. The most common types are in-line engines, V-engines and boxer engines. The various car engines can differ in terms of their power-to-weight ratio, with the advantage of designs that have a low mass and require little material. In this respect, the frequently used in-line engine can score points. This type of combustion engine has a compact design and low engine weight, while the V-engine requires more material due to its design characteristics. However, this type can also have a good power-to-weight ratio with a short design. For boxer engines, the power-to-weight ratio depends heavily on the particular design and materials used.

The bore to stroke ratio

The ratio between the cylinder bore and the stroke of the cylinder influences the characteristics of engines. This affects both the speed characteristics and the overall performance of the engine. Internal combustion engines are divided into short-stroke, square-stroke and long-stroke engines. In the so-called short-stroke engines, the piston stroke is smaller than the cylinder bore. These short-stroke engines are well suited to high engine speeds and achieve high engine output while maintaining high gas throughput. In long-stroke engines, the stroke is larger than the bore. The so-called long-stroke engines develop high torque even at low engine speeds. If the stroke and cylinder diameter are the same size, the engine is referred to as a quadrathuber due to its square design.

The compression ratio of the engine

Another characteristic of internal combustion engines is the compression ratio. This indicates the degree to which the charge is compressed within the cylinders. In the four-stroke engines commonly used in automobiles today, this takes place in the compression stroke, with efficiency and power output increasing as the compression ratio rises, especially in gasoline engines. Higher compression often results in better combustion with lower pollutant emissions. However, increasing the compression ratio in gasoline engines is not possible without limits. If the compression ratio is too high, the mixture could spontaneously ignite in the cylinders, resulting in what is known as knocking. Today, however, modern engines with direct injection and integrated knock sensors allow relatively high compression ratios.

Engine performance and variable valve timing

Variable valve control systems allow optimized filling of the combustion chambers with air-fuel mixture. In this context, variable valve timing allows the valve lift or opening duration to be changed even while the engine is running. Variable valve timing enables optimized torque even at low engine speeds while maintaining high efficiency. At high engine speeds, power output can also improve. The exhaust gas behavior of internal combustion engines also improves when variable valve timing is used. Many engines with variable valve timing also do not require a throttle valve in normal operation.