In this article, I will be giving a basic overview of how the rotating assembly in a piston engine makes usable power from burning fuel and air in the cylinders.
Table of Contents
Several assumptions will be made in this article. Friction will be ignored, as will the effects of the mass of moving parts, and so forth. Most statements will be based on ideal cases.
Engine displacement is the measure of the volume of the cylinders. The volume of a cylinder in an engine is found using the equation below.
The displacement of a cylinder is equal to the square of the bore multiplied by the stroke and by π over four.
Finding the overall engine displacement is as simple as multiplying the displacement of a single cylinder by the number of cylinders in the engine.
The distance between the centre of the connecting rod and the centre of the crankshaft is known as the crank pin offset and is equal to one-half of the stroke.
The crank pin offset is equal to ½ of the stroke.
The piston face area is simply the area of a circle with the diameter equal to the cylinder bore.
The area of the piston face is equal to the square of the bore multiplied by π over four.
The pressure in the cylinder comes from the fuel and air mixture being burned. If a dense charge of fuel and air can be drawn into the cylinders on each cycle, a high cylinder pressure will be made.
The cylinder pressure acts on the area of the face of the piston, resulting in a force which is applied downward to the connecting rod, which is attached to the crankshaft.
It can be seen from the diagram below that the combination of cylinder pressure, piston face area, and crank pin offset determines the torque. Increasing any of these three things will increase the torque on the crankshaft.
The torque on the crankshaft varies with the product of the cylinder pressure and displacement.
Generally speaking, it is simplest to increase torque output by increasing the displacement of the engine. It is more difficult and costly to design cylinder heads and other components to create more cylinder pressure than to simply cast larger pistons or a crankshaft with a larger stroke. The phrase "No replacement for displacement" is based on this fact.
Effect of Bore and Stroke on Torque
Torque applied to the crankshaft comes directly from the product of the cylinder pressure and overall displacement, and is not theoretically affected by the bore and stroke dimensions, as seen in the example below.
Both cylinders displace a total of 500cc. The first has a bore and stroke of 83mm x 92mm, making it undersquare. The second cylinder has a bore and stroke of 92mm x 75mm, making it oversquare. Each cylinder will have an arbitrary pressure, P, applied to the piston.
The bore and stroke alone do not have any effect on the torque produced from an arbitrary cylinder pressure.
While the torque produced by the different sized cylinders is equal because the overall displacement was the same, the forces were not. The oversquare cylinder will have more force applied to the piston than the undersquare cylinder, while the undersquare cylinder has more leverage on the crankshaft than the oversquare cylinder. The product of the force and lever arm distance ends up being the same, which will always hold true when the overall displacement is the same for both cylinders.
The real-life effects of different bore and stroke combinations will be covered in a separate article.
Most piston engines have multiple cylinders, each on a different phase of the four-stroke cycle.
If two engines with differing numbers of cylinders have the same overall displacement and are given the same cylinder pressure, the average torque created for every two rotations of the crankshaft will be the same.
For example, on a 2.4L six-cylinder engine, each cylinder is 400cc, which is smaller than a 2.4L four-cylinder, where each cylinder would be 600cc. Each individual cylinder on the six-cylinder engine would be making 50% less torque than each individual four-cylinder, but that torque would be made 50% more often.
The only difference between the above two engines would be the shape of the torque curve when measured continuously for two revolutions of the crankshaft. The four-cylinder would have much higher torque peaks when each cylinder fires, but then much lower valleys when there are no cylinders on a significant part of the power cycle. On the six-cylinder engine, the peaks and valleys would be much flatter, with the average torque being the same for both engines. Below is a very simple illustration of what the torque measurements might look like.
A six-cylinder will have a flatter torque graph than a four-cylinder of the same displacement, but the average torque will be the same.
Revs is a colloquial term for angular velocity, which is the rate at which something as rotating. In reference to a piston engine, it is the angular velocity of the crankshaft. The most common units for angular velocity with respect to piston engines are revolutions per minute or RPM, which is where the term revs comes from.
An engine's ability to rev at a high rate is determined by numerous factors. For one, the moving parts must be able to withstand the stresses of high-speed operation, and be relatively lightweight to reduce the stresses that they apply on supporting components. For this reason, building an engine that can rev very high can become costly due to the requirement for components to be made from exotic materials and to exacting tolerances. It is for this reason that engines which can rev extremely high are not found in regular production road cars.
Fast-moving parts are also more susceptible to wear. An engine revving at a constant 4000RPM would, in theory, suffer twice the wear in the same period of time as an engine revving at only 2000RPM.
Power output is not affected by the specific amount of torque or revs, but by their product. An engine making 300lb-ft of torque at 4000RPM produces the same amount of power as an engine making 400lb-ft of torque at 3000RPM (228hp).
The force generated by the cylinder pressure on the piston face along the distance of the crank pin offset creates torque. The rotational speed (revs) is the rate of torque production, which is the definition of power on a rotating shaft (the crankshaft).
Power is made from applying pressure to the piston face, which is offset from the crankshaft's center, which generates torque. The torque multiplied by the rotational speed of the engine is the power made.
Intuitively, the more fuel and air that is burned in a given period of time (higher revs), the more power will be made. Also, if the fuel and air mixture can be burned more effectively (resulting in a higher cylinder pressure), or if the losses due to friction in the engine can be reduced, more power will result. Generally speaking, engine power is increased mostly by means of higher revs and increased cylinder pressure.
In automotive press, it is common to refer to specific output, which is the amount of peak power that an engine makes per unit of displacement. The most frequently used units are horsepower per litre (hp/L), which is a classic example of the metric and imperial system butting heads.
Since the specific output value of an engine does not take into account any of the other properties of the engine, it cannot be used to accurately determine the engine's fuel economy (mileage), fuel efficiency (specific fuel consumption), weight, reliability, or any other measure of the engine's greatness. Also, the specific output is only calculated with the engine's peak power value, which ignores the engine's average power production across its entire rev range.
The only engine property that can be partially derived from specific output values is the engine's ability to rev. This is because a high-revving engine will be making more power from a given amount of torque, which is directly proportional to the engine's displacement. A low-revving engine is not necessarily inferior in any way compared to a high-revving engine making similar power. In fact, high-revving engines tend to have narrower power bands, so an engine with a high specific output may have a narrow power band. In the case of the Honda S2000, which has an engine that is known for its high specific output, the assumption of a narrow power band holds true.
Further, small engines are often capable of revving higher than a large engine, as most of the moving parts are significantly smaller and therefore lighter. As a result, most small engines have a higher specific output than most large engines, though the power output and power-to-weight ratio is lower in most cases.
Forced induction can significantly increase specific output, because the cylinder pressure will be much higher, resulting in higher power without large displacement. It is possible to achieve very high specific output with high boost pressures from turbochargers and superchargers.