A four stroke or four cycle engine is an engine operating on the principle of internal combustion, in which pistons (one per cylinder in multi-cylinder engines) completes four separate strokes for each thermodynamic cycle, which is accomplished over two consecutive revolutions of the engine’s crankshaft.
These four strokes are: the Intake stroke, the Compression stroke, the Power stroke and the Exhaust stroke. The sectional diagrams below illustrate these four strokes in a single-cylinder engine; the curved red arrows represent the direction of rotation of the camshafts (small arrows) and the engine’s crankshaft (larger arrows). The blue-colored valve is the inlet valve; the pink-colored valve is the exhaust valve. The straight red arrows show the up or down direction of movement of the piston.
Most four-stroke engines run on either gasoline or similar fuel (including biofuels) or on diesel fuel and operate in a similar manner.
This article describes the operation of a gasoline powered engine. This type of engine is used in a wide range of applications, including automobiles, boats, motorcycles, generator sets, lawnmowers, etc, etc.
For the purposes of this description the cycle begins with piston at its highest position in the cylinder (so the connecting rod between the piston and the engine’s crankshaft is vertical, as shown in the Starting Position diagram above).
That position – with the piston at its highest level in the cylinder – is referred to as Top Dead Center (TDC). When the piston is at the lowest point of its travel, that position is known as Bottom Dead Center (BDC).
Annotations on the following diagram (a repeat of the Starting Position diagram above), identify the various engine components described in this article:
- The light green areas in the above diagram indicate the cavities cast into the cylinder head and engine block for water cooling. In some engines, these could be replaced by fins cast onto the outsides of the cylinder head and engine block if the engine is an air-cooled type.
- The camshafts – one for the inlet valve(s) and one for the exhaust valve(s) in the engine illustrated – rotate at half the speed of the crankshaft.
- For many years the engine blocks and cylinder heads were made from cast iron. In recent times, however, improved technology and manufacturing techniques have facilitated engines being made mostly from aluminium alloys, thus saving weight and thereby improving the power-to-weight ratio.
- The Intake Stroke
- The Compression Stroke
Both inlet and exhaust valves remain closed as the crankshaft’s continuing rotation brings the piston back towards TDC, compressing the just-introduced fuel/air mixture in the upper part of the cylinder and the cast cavity in the cylinder head around the valves and spark plug – usually called the combustion chamber.
- The Power Stroke
This stroke is at the beginning of the second revolution of the crankshaft and therefore of the engine. With the piston at or close to TDC, the compressed and therefore heated fuel/air mixture is ignited by the spark plug, which causes an explosion of the mixture, forcing the piston back down the cylinder towards BDC. Both inlet and exhaust valves remain closed during the power stroke.
- The Exhaust Stroke
The exhaust valve is opened by action of a cam on the exhaust camshaft as the piston passes BDC and begins to rise up the cylinder once more. The combustion products (gases) are forced out through the exhaust valve and exhaust port as the piston approaches TDC, as represented by the pink color in the diagram labelled 4. Exhaust Stroke. This is the end of the four stroke cycle.
- How the Power is Generated
Firstly, it is important to note that in an internal combustion engine, the reciprocating linear motion of the piston (up and down) is converted to rotational motion by the engine’s crankshaft. Also, a reminder that it is only one of the four strokes – the power stroke – that actually generates the power. A power stroke occurs every two rotations of the engine crankshaft.
Internal combustion engines – irrespective of the number of cylinders they have – are fitted with a heavy metal flywheel on the end of the crankshaft. Usually made of machined steel, the flywheel mass stores kinetic energy and maintains the inertia of the engine revolving through its cycles.
As the power generated by the combustion of fuel in the cylinder(s) is intermittent, i.e. once every power stroke per cylinder, the energy stored in the flywheel helps to maintain and smooth out the rotation of the engine crankshaft created by the individual impulses resulting from the power strokes within the cylinder(s).
In multi-cylinder engines, e.g. engines having up to (say) 12 cylinders, all pistons and connecting rods are connected to a common crankshaft to which the flywheel is fitted, external to the engine block. The flywheel is the interface to the vehicle’s transmission system which comprises a clutch (to disconnect the engine momentarily from the transmission to allow gear changes) and some sort of gearbox. In automatic transmission systems the clutch operates automatically when needed.
For other applications of internal combustion engines like portable generator sets, the flywheel is usually mounted between the engine and the generator, linking the engine crankshaft and the generator shaft on a concentric axis.
- Power Output Constraints
There are many factors affecting the amount of power than can be extracted from an internal combustion engine of a specific bore (cylinder diameter), stroke (distance between TDC and BDC), and number of cylinders.
As the combustible material used is a fuel/air mixture, one important influence on the power developed by a given engine is the efficiency of the system used to mix the fuel and air as it enters the engine. Such devices are commonly carburetors, which typically operate in a passive manner and have an air valve in a passage through which the intake air passes as it is drawn into the engine on the Intake stroke. See diagram below:
The setting of the valve (closed, open or somewhere in between) is set by a throttle or accelerator control, which may be preset in (say) a generator or lawnmower application, or constantly variable for an automobile application. The more the air valve is open, the more fuel/air mixture is drawn into the engine, allowing speed to increase and develop more power.
In more sophisticated and more modern engines, the carburetor may be replaced by a fuel injection system, usually electronically controlled, that injects a precisely metered amount of fuel into the cylinder. This method not only increases efficiency and power, but reduces exhaust emissions by ensuring that the mixture produces minimal unwanted residues.
For even more power output, the engine can be fitted with a supercharger or a turbocharger. Both operate on the principle of compressing air before it enters the engine, allowing a greater volume of fuel/air mixture to be burnt for each power stroke. A supercharger is a compressor driven directly off the engine (e.g. via a belt from a crankshaft-mounted pulley), whereas a turbocharger is a compressor driven by a turbine inserted into the exhaust system (external to the engine) and powered by the exhaust gases passing from the engine out to atmosphere.
Compression ratio of the engine also affects power output. The compression ratio is the ratio of the volume of the combustion chamber with the piston at BDC to the volume when the piston is at TDC. The higher this ratio the more power can be developed. However, to operate at higher compression ratios, engines need higher octane fuel to run properly.
Other factors affecting the power available from an engine are basic design, such as the ratio of the cylinder bore to the stroke, the smoothness of the internal inlet and exhaust ports (to ease gas flow), sizes of the inlet and exhaust valves, etc. Also, the ignition timing (the actual instant in the transition between the compression stroke and the power stroke when the fuel/air mixture is ignited), is critical. If it is too early, there can be a phenomenon called pre-ignition which causes a knocking noise from the engine. If it too late (called retarded ignition timing), the engine power is reduced. In most cases the system is set to provide the spark a fraction before TDC, which has been found to be the optimum setting for best power output.
- Global Utilization of Four Stroke Engines
Undoubtedly, these engines have had a major impact on our world. For example, they have for the best part of a century been the dominant engine type for automobiles, which have given man – in the developed countries at least – the means of independent, privately owned, mass personal transport. That affected our culture in less obvious ways. For instance, people could travel whenever they wanted and wherever they wished and at greater speed than before. Previously, the great majority of the population had not travelled more than a few miles from where they were born. It also meant that people could choose to live further from their place of work encouraging the development of suburbia. In many ways the world had become a smaller place.
Imagine a world where all fresh produce had to locally grown, so no “out of season” fruit and vegetables. There would probably be very few roads, too – remember most roads were built specifically for the use of automobiles. Even our industry has been shaped by the automobile as it was in automobile manufacture that the production line was first created.
However, it is not all good news. Burning of fossil fuels including gasoline is thought to cause climate change or global warming, although there are prospects of alternative fuels being developed to partially offset that particular problem.
As well as powering our automobiles, four stroke internal combustion engines are also widely used for many other transport or vehicle applications such as buses, trains, freight, industry (like fork lift trucks), construction (diggers, cranes, etc), pleasure boats and merchant and cruise ships and more.
Just think how different today’s society would be without a comprehensive transport network, and the motorized machinery used in industry, construction and agriculture. Even our fishing fleets use boats powered by internal combustion engines.
The four stroke engine has also been extensively utilized for non-mobile applications such as powering generators that provide emergency electric power, powering water pumps for irrigation, lawnmowers and more. Until the 1950’s brought jet engine-powered passenger aircraft, all passenger aircraft were also powered by gasoline-fueled engines.
What if we had never had internal combustion engines? Although they are popularly considered to be a major source of global warming, it has been claimed (in an article in the British Independent newspaper) that “livestock are responsible for 18 percent of the greenhouse gases that cause global warming, more than cars, planes and all other forms of transport put together.” Their digestive systems and consequent flatulence are the problem.
Another source – this time Robert Fogel, a well known historian, claimed that at the start of the 20th century there were 200,000 horses in New York City alone. Because of the pathogens contained in the resulting huge amounts of horse manure, the automobile was at the time of its inception seen as the solution to that problem, helping to produce a cleaner and healthier environment. Even outside of the cities, air and water pollution caused by animals in agricultural areas (e.g. pigs, cows, chickens, etc) had greatly added to the pollution problem.
Of course pollution from the exhausts of internal combustion engines is a real problem; that cannot be denied. However, in the area of agriculture alone, the internal combustion engine has brought farm machinery that allows many more people to be fed from the crops grown, whilst eliminating large numbers of greenhouse gas-producing animals (and growing the feed that they would have consumed). It is also noteworthy that the methane produced by animals is a gas with much greater “greenhouse” potency than the CO2 produced by gasoline-fueled engines. Essentially, the internal combustion engine is less polluting than the animals it replaced.