A Brief Note on Internal Combustion Engines
Before we start with two stroke and four stroke engines and compare 2 stroke vs 4 stroke engines with respect to important parameters of an engine, let us quickly take a look at what exactly is an Engine and also what is an Internal Combustion Engine. Most engines we have are technically ‘Heat Engines’ as they convert thermal energy (usually from combustion of fuels, so chemical energy to thermal energy and) into mechanical work. In external combustion engines, the combustion of fuel takes place outside the main engine parts (the cylinder). Steam Engine is the best example of external combustion engine. In case of an internal combustion engine, the combustion of fuel happens inside the cylinder. All our automobile and generator engines are of this type. The power to weight ratio is very less in case of internal combustion engines. This made it possible to manufacture reasonably small engines with decent power output that we can use in our vehicles. Since their first development in the second half of the 19th century, the Internal Combustion Engines were always a topic of interest to engineers and scientists. The first generation of internal combustion engines (J. J. E. Lenoir developed them in mid-1800’s) were having a poor efficiency of 4 – 5%. Couple of years later, Nikolaus August Otto and Eugen Langen developed an improved version of the Lenoir engine with efficiency up to 11%. This Otto – Langen engine later became the “Otto Cycle” engine, which is nothing but a spark ignition engine (all modern gasoline/petrol engines). All these engines are technically four stroke engines. During 1880’s, development of lighter and mechanically simple two stroke engines picked up its pace. In order to further increase the efficiency of the Otto engine, Rudolf Diesel started working on the fuel as well the how engine compresses the fuel. This is the Diesel Engine that used Diesel Fuel, where it can ignite the fuel with high compression (and no need for any sparks). In contrast to the Otto Cycle engines, which need a spark to combust the fuel, the “Diesel Cycle” engines simple work on compressing the fuel to ignite it. Both the Otto Cycle Engine (gasoline/petrol engines that have combustion at constant volume air/fuel using a spark ignition) or the Spark Ignition (SI) Engine and the Diesel Cycle Engine (diesel fuel engines that have combustion at constant pressure air/fuel using compression of this mixture to ignite the fuel) or the Compression Ignition (CI) Engine are available as both 2 Stroke and 4 Stroke Engines. But what exactly is a Two Stroke Engine or a Four Stroke Engine? How can we compare 2 Stroke vs 4 Stroke Engines? What are their advantages and disadvantages? Let us find out now. There are a lot more things to learn about Internal Combustion Engines such as its parts, terminology, working, classification, types and many more. Since this guide is focusing on the Two Stroke vs Four Stroke Engines, we decided not to go in detail about those topics. But, if you are interested, we can make a series of guides on Internal Combustion Engines.
What is a 4-Stroke Engine?
Before understanding what is a 4-stroke engine or 2-stroke engine, let us first see what exactly is a stroke. We know that an engine consists of a cylinder and a piston that moves up and down in this cylinder (again, this is a very broad image of the internals of an engine). Stroke is the distance a piston covers between the upper and lower limits in either direction in the cylinder. This distance is an important parameter in the working of the engine along with other dimensions such as bore (the diameter of the cylinder), piston area and the stroke to bore ratio. Now that we know what a stroke is, we will now move on to the Four Stroke Engine. For an engine to work properly, it must follow a strict and sequential cycle of operations. This applies to both SI and CI Engines. A 4 Stroke Engine completes its cycle of operation in four strokes or two crankshaft revolutions. Each stroke rotates the crankshaft by 180° or half turn. Therefore, four stoke cycle completes 720° of crankshaft rotation or two full turns. What is the Cycle of Operation in a 4 Stroke Engine? The cycle of operations in an ideal 4-stroke engine has the following four strokes:
Intake (or Suction or Induction) Stroke Compression Stroke Power (or Expansion) Stroke Exhaust Stroke
Image Both the SI and CI Four Stroke Engines follow this cycle of operations but their implementations are slightly different (as SI uses a spark plug to ignite the fuel and CI uses just compression to ignite the fuel). One important difference is the compression ratio. It is significantly higher in CI engines than SI engines. There are other differences in individual strokes and we will see them in the working.
Working of a Four Stroke Engine
In order to understand the working of a four-stroke engine, you simply need to understand the cycle of operation and what happens in each stroke.
Intake Stroke
This stroke starts when the piston is at the TDC (Top Dead Center – when piston is at the upper most position during its travel, it is known as TDC) and is about to move downwards. During this time, the inlet valve opens up while the exhaust valve closes. If the engine has a carburetor, then the suction of the piston causes the mixture of fuel and air to be drawn into the cylinder. In case of a fuel injection mechanism, then a computer-controlled injector will spray the fuel. This is true for an SI Engine. In case of a CI Engine, the suction draws only the air. The piston will reach BDC (Bottom Dead Center – when the piston is at the lower most position during its travel, it is known as BDC) and at the end of intake stroke, the inlet valve closes.
Compression Stroke
In this stroke, the piston moves from BDC to TDC and during this process, it compresses the air fuel mixture (in case of SI engine) or just the air (in case of CI engine) that is taken during the intake stroke. In compression stroke, both the inlet and exhaust valves remain closed. At the end of the compression stroke i.e., when the piston is back at the TDC (or close to it), a spark plug will ignite the compressed air-fuel mixture (in case of SI engine) or injects fuel (in case of CI engine). This action will combust the fuel and produces heat energy with temperatures rising to 2,000°C or more. Due to the nature of fuel in CI engine (which is Diesel fuel), when the fuel comes in contact with the compressed air, it starts combustion.
Power Stroke
Due to the high pressure created at the end of the compression stroke from burning fuel, the piston moves to BDC. Even in this stroke, both the valves (inlet and exhaust) remain closed. The engine produces power only in this stroke. As the piston goes from TDC to BDC, pressure and temperature in the cylinder gradually decreases.
Exhaust Stroke
At the end of the power stroke, the exhaust valve opens and the piston starts moving from BDC to the TDC in the exhaust stroke. In this process, it pushes the burnt gases from the cylinder through the exhaust valve. When the piston is near TDC, the exhaust valve closes and the inlet valve opens, once again starting the intake stroke.
Applications of a 4-Stroke Engine
The most common and most popular application of four-stroke engines is automobiles. You can find small 4 Stroke Engines that produce about 30 to 60kW (40 to 80HP) at 4,500 RPM in mopeds, scooters and motorcycles. Cars, buses and trucks also use slightly larger 4-Sroke engines that produce 90kW or more power. Apart from automobiles, you can also find four stroke engines in mobile generators, aircrafts, pump sets, construction machines, power tools, air compressors, drilling rigs, tractors, etc.
What is a 2-Stroke Engine?
If you observe the working of a four-stroke engine, it is clear that out of the four strokes, only one stroke (the power stroke) produces power and also there are two non-productive strokes in the form of Intake and Exhaust strokes. As a result, the engine is producing usable power for every two revolutions of the crankshaft. What if we could somehow alter this arrangement without changing the position of the piston and achieve power stroke for every revolution of the crankshaft? This is the same idea that Dugald Clerk, a Scottish engineer got and started working on a new type of engine, which later became the 2 Stroke Engine. A Two Stroke engine completes its cycle of operations in just two strokes or one revolution of the crankshaft. The two strokes are Compression and Power. But if there are no Intake and Exhaust strokes, how will the engine get fuel/air and how will it pump out the burnt gases? 2 Stroke Engine achieves this with a simple design. It uses the air/fuel mixture compressed in crankcase to fulfil the intake process and as the compressed fuel enters the cylinder, the low pressure burnt gases moves out through an exhaust port.
Working of a Two Stroke Engine
The construction of a 2-Stroke engine is relatively very simple when we compare it to a 4-stroke engine. They also have high power to weight ratio with few moving parts. But how does a 2 Stroke Engine work? Let us see now. One of the simplest and popular 2-stoke engine designs is the crankcase scavenged engine. In this design, the bottom surface of the piston and the portion of the crankcase beneath the cylinder acts as a scavenging pump. During the compression stroke, as the piston moves from BDC to TDC, spring loaded inlet valve in the crankcase intakes the air/fuel mixture due to the decrease in pressure in the crankcase. At the end of the compression stroke, the ignition of fuel happens (either using a spark plug or just due to the compression). Now, the pressure will start the Power Stroke and as the piston moves from TDC to BDC, the air/fuel mixture in the crankcase gets compressed. Near the end of the power stroke, the piston will unblock the exhaust port and the burnt gases leave the cylinder. As the piston moves down during the power stroke, it also unlocks a Transfer port, which is responsible for the compressed air/fuel mixture in the crankcase to enter the cylinder (as the piston moves upwards in at the beginning of the next compression stroke).
Applications of a 2-Stroke Engine
Once upon a time, small 2 stroke engines are the primary choice in mopeds, scooters, motorcycles, lawn mowers, and slightly larger engines for motor boats and for ship propulsion. But due to their high fuel consumption, loss of fuel in the process of scavenging and general pollution concerns, the production of two stroke engines is almost extinct.
Comparison: 2 Stroke vs 4 Stroke Engines
Let us now see the comparison of 2 Stroke vs 4 Stroke Engines. We have taken some important aspects of an engine and compared them for both Two Stroke and Four Stroke Engines.
Conclusion
Internal Combustion Engines are an integral part of modern automobiles, power tools and power generation. The main classification of Internal Combustion Engines is based on the number of strokes (or the number of crankshaft revolutions) an engine takes. In this, we have the Four-Stroke and Two-Stroke Engines. We saw what is a four-stroke engine and its working. We also saw how a two stroke engine works. Finally, we drew up a comparison table of 2 stroke vs 4 stroke engines. Comment * Name * Email * Website
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