The Working Cycle
Stroke:
"Stroke" refers to the movement of the piston in the engine. 2 Stroke means one stroke in each direction. A 2 stoke engine will have a compression stroke followed by an explosion of the compressed fuel. On the return stroke new fuel mixture is inserted into the cylinder.
A 4 stroke engine has 1 compression stroke and 1 exhaust stroke. Each is followed by a return stroke.The compression stroke compresses the fuel air mixture prior to the gas explosion. The exhaust stroke simply pushes the burnt gases out the exhaust.
FOUR STROKE PETROL ENGINE
During the intake stroke, the piston moves downward, drawing a fresh charge of vaporized fuel/air mixture. The illustrated engine features a poppet intake valve which is drawn open by the vacuum produced by the intake stroke. Some early engines worked this way; however, most modern engines incorporate an extra cam/lifter arrangement as seen on the exhaust valve.
Compression:
As the piston rises, the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture.
At the top of the compression stroke, the spark plug fires, igniting the compressed fuel. As the fuel burns it expands, driving the piston downward.
Exhaust:
At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder.
The primary differences between a diesel engine and a gasoline engine are as follows:
TWO STROKE PETROL ENGINE
Intake:
Crankcase compression
Transfer/Exhaust
Power stroke:
At the top of the stroke, the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle. (At the same time, another crankcase compression stroke is happening beneath the piston.)
Since the two stroke engine fires on every revolution of the crankshaft, a two stroke engine is usually more powerful than a four stroke engine of equivalent size. This, coupled with their lighter, simpler construction, makes the two stroke engine popular in chainsaws, line trimmers, outboard motors, snowmobiles, jet-skis, light motorcycles, and model airplanes.
Unfortunately, most two stroke engines are inefficient and are terrible polluters due to the amount of unspent fuel that escapes through the exhaust port.
Two Stroke Diesel Engine
It turns out that the diesel approach, which compresses only air and then injects the fuel directly into the compressed air, is a much better match with the two-stroke cycle. Many manufacturers of large diesel engines therefore use this approach to create high-power engines.
The figure below shows the layout of a typical two-stroke diesel engine:
Two Stroke Diesel Engine For a Ship.
The two-stroke diesel cycle goes like this:
Two Stroke Diesel Engine
Which is Better, a 2 Stroke or 4 Stroke Engine?
Every website you look at regarding 2 stroke and 4 stroke engines seems to be comparing the pros and cons of each engine. But is such a list really fair or just a comparison between a cheaply made 2 stroke and an expensive 4 stroke that has had lots of time and research put into it?
A Common List of Advantages and Disadvantages
Advantages of 2 Stroke Engines:
- Two-stroke engines do not have valves, simplifying their construction.
- Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost.
- Two-stroke engines are lighter, and cost less to manufacture.
- Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.
Clearing the Air
Most of what is written on advantages and disadvantages of 2 strokes Vs 4 strokes is not actually correct.
Finally, the arguments of simplicity, weight, power to weight, and cost of manufacturing are not a function as such of 2 stroke versus 4 stroke engines. The mistake of most of these commentaries is that they are comparing a simple chainsaw 2 stroke engine with a complex 4 stroke engine from a automobile - not a very fair comparision.
As far as the exhaust emissions of 2 strokes - check out the Surrich/Orbital 2 stroke design that Mercury outboards are using - this is as clean burning as any 4 stroke.
The ONLY correct comparison of 2 strokes with 4 strokes is that a 2 stroke can (in theory) produce twice the power of a 4 stroke for the same sized engine and the same revs.
The Better One
At the end of the day the winner is probably going to be the one that has had more money and technology spent on it-The four stroke engine. In these days of quick and cheap international production schedules you can't take it for granted that the 4 stroke will be better. So for your particular application, line up the options and make a decision based on what's available, not based on lists that miss the key points of difference..
Stroke:
If an engine has to work successfully then it has to follow a cycle of operations in a sequential manner. The sequence is quite ridged and it cannot be changed.
"Stroke" refers to the movement of the piston in the engine. 2 Stroke means one stroke in each direction. A 2 stoke engine will have a compression stroke followed by an explosion of the compressed fuel. On the return stroke new fuel mixture is inserted into the cylinder.
A 4 stroke engine has 1 compression stroke and 1 exhaust stroke. Each is followed by a return stroke.The compression stroke compresses the fuel air mixture prior to the gas explosion. The exhaust stroke simply pushes the burnt gases out the exhaust.
A 4 stroke engine usually has a distributor that supplies a spark to the cylinder only when its piston is near TDC (top dead center) on the fuel compression stroke, ie. one spark every two turns of the crank shaft.
Some 4 stroke engines do away with the distributor and make sparks every turn of the crank. This means a spark happens in a cylinder that just has burnt gasses in it which just means the sparkplug wears out faster.
The IC engines are classified into petrol engines (four stroke and two stroke) & diesel engines (four stroke and two stroke).
The credit for inventing a spark ignition engine goes to Nicolaus A.Otto (1876) and the compression ignition engine was invented by Rudolf Diesel (1892). So the engines are referred as Otto engine & Diesel engine.
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| Nicolaus A. Otto |
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| Rudolf Diesel |
FOUR STROKE PETROL ENGINE
A four stroke petrol engine can be otherwise called as four stroke spark ignition engine since the engine uses spark plugs to ignite the charge (air- fuel mixture). In a four stroke engine, the cycle of operations is completed in four strokes of the piston or two revolutions of the crankshaft. The cycle of operations are explained below.
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| The Four Stroke Cycle. |
Intake:
During the intake stroke, the piston moves downward, drawing a fresh charge of vaporized fuel/air mixture. The illustrated engine features a poppet intake valve which is drawn open by the vacuum produced by the intake stroke. Some early engines worked this way; however, most modern engines incorporate an extra cam/lifter arrangement as seen on the exhaust valve.
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| The Intake Stroke. |
As the piston rises, the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture.
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| The Compression Stroke. |
Power:
At the top of the compression stroke, the spark plug fires, igniting the compressed fuel. As the fuel burns it expands, driving the piston downward.
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| The Power Stroke. |
Exhaust:
At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. The upward stroke of the piston drives the exhausted fuel out of the cylinder.
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| The Exhaust Stroke. |
Four Stroke Cycle Animation
The Four Stroke Cycle
Four-Stroke Diesel Engine
The four stroke cycle of diesel engine is similar to the four stroke cycle of the petrol engine. The main contrast between both the engines is that the petrol engine uses a spark plug to ignite the fuel and air mixture whereas in diesel engine fuel injectors are used to inject the fuel directly into the combustion chamber. The compression ratio of the diesel engine is very high which increases the air temperature inside the cylinder self sufficient to ignite the fuel when it is injected.
Here only the air is compressed in the cylinder and no air fuel mixture is supplied into the cylinder. During the compression stroke the temperature of the air inside cylinder is high enough to ignite the fuel when it is injected. so here fuel is supplied directly.
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| The Four Stroke Diesel Cycle |
The four-stroke diesel engine is similar to the four-stroke gasoline engine. They both follow an operating cycle that consist of intake, compression, power, and exhaust strokes. They also share similar systems for intake and exhaust valves.
1. The fuel and air mixture is ignited by the heat generated by the compression stroke in a diesel engine versus the use of a spark ignition system on a gasoline engine.
2. The fuel and air mixture in a diesel engine is compressed to about one twentieth of its original volume, while in a gasoline engine the fuel and air mixture is only compressed to about one eighth of its original volume. The diesel engine must compress the mixture more tightly to generate enough heat to ignite the fuel and air mixture.
3. The gasoline engine mixes the fuel and air before it reaches the combustion chamber. A diesel engine takes in only air through the intake port. Fuel is put into the combustion chamber directly through an injection system. The air and fuel then mix in the combustion chamber.
4. The engine speed and the power output of a diesel engine are controlled by the quantity of fuel admitted to the combustion chamber. The amount of air is constant. On the gasoline engine, the speed and power output is regulated by limiting the air and fuel mixture entering the engine .
A diesel engine is much more efficient than a gasoline engine, such as the diesel engine does not require an ignition system due to the heat generated by the higher compression, the diesel engine has a better fuel economy due to the complete burning of the fuel, and the diesel engine develops greater torque due to the power developed from the high-compression ratio. The strokes that make up the four-stroke cycle of a diesel engine follow.
The piston is at top dead center at the beginning of the intake stroke, and, as the piston moves downward, the intake valve opens. The downward movement of the piston draws air into the cylinder, and, as the piston reaches bottom dead center, the intake valve closes. When the piston reaches the top dead centre the air is compressed which is self sufficient to ignite the fuel when it is injected into the combustion chamber through a fuel injector.
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| A Diesel Engine |
TWO STROKE PETROL ENGINE
The two stroke engine employs both the crankcase and the cylinder to achieve all the elements of the Otto cycle in only two strokes of the piston. In two stroke engines the cycle is completed in one revolution of the crankshaft. Dugald Clark invented the two stroke engine. The cycle that takes place in a two stroke engine as follows. 1. The intake 2. The power stroke. The remaining process takes place within the two cycles are explained below.
Intake:
The fuel/air mixture is first drawn into the crankcase by the vacuum that is created during the upward stroke of the piston. The illustrated engine features a poppet intake valve; however, many engines use a rotary value incorporated into the crankshaft.
Crankcase compression
During the downward stroke, the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. (At the same time, another intake stroke is happening beneath the piston).
Transfer/Exhaust
Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well.
Power stroke:
At the top of the stroke, the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle. (At the same time, another crankcase compression stroke is happening beneath the piston.)
Since the two stroke engine fires on every revolution of the crankshaft, a two stroke engine is usually more powerful than a four stroke engine of equivalent size. This, coupled with their lighter, simpler construction, makes the two stroke engine popular in chainsaws, line trimmers, outboard motors, snowmobiles, jet-skis, light motorcycles, and model airplanes.
Unfortunately, most two stroke engines are inefficient and are terrible polluters due to the amount of unspent fuel that escapes through the exhaust port.
The Two Stroke Cycle
Two Stroke Diesel Engine
It turns out that the diesel approach, which compresses only air and then injects the fuel directly into the compressed air, is a much better match with the two-stroke cycle. Many manufacturers of large diesel engines therefore use this approach to create high-power engines.
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| Two Stroke Diesel Engine |
The figure below shows the layout of a typical two-stroke diesel engine:
The Strokes of 2-stroke Diesel Engine
At the top of the cylinder are typically two or four exhaust valves that all open at the same time. There is also the diesel fuel injector (shown above in yellow). The piston is elongated, as in a gasoline two-stroke engine, so that it can act as the intake valve. At the bottom of the piston's travel, the piston uncovers the ports for air intake. The intake air is pressurized by a turbochrager or a supercharger (light blue). The crankcase is sealed and contains oil as in a four-stroke engine.
The two-stroke diesel cycle goes like this:
- When the piston is at the top of its travel, the cylinder contains a charge of highly compressed air. Diesel fuel is sprayed into the cylinder by the injector and immediately ignites because of the heat and pressure inside the cylinder.
- The pressure created by the combustion of the fuel drives the piston downward. This is the power stroke.
- As the piston nears the bottom of its stroke, all of the exhaust valves open. Exhaust gases rush out of the cylinder, relieving the pressure.
- As the piston bottoms out, it uncovers the air intake ports. Pressurized air fills the cylinder, forcing out the remainder of the exhaust gases.
- The exhaust valves close and the piston starts traveling back upward, re-covering the intake ports and compressing the fresh charge of air. This is the compression stroke.
- As the piston nears the top of the cylinder, the cycle repeats with step 1.
From this description, you can see the big difference between a diesel two-stroke engine and a gasoline two-stroke engine: In the diesel version, only air fills the cylinder, rather than gas and air mixed together. This means that a diesel two-stroke engine suffers from none of the environmental problems that plague a gasoline two-stroke engine. On the other hand, a diesel two-stroke engine must have a Turbocharger or a supercharger, and this means that you will never find a diesel two-stroke on a chain saw-- it would simply be too expensive.
Which is Better, a 2 Stroke or 4 Stroke Engine?
Every website you look at regarding 2 stroke and 4 stroke engines seems to be comparing the pros and cons of each engine. But is such a list really fair or just a comparison between a cheaply made 2 stroke and an expensive 4 stroke that has had lots of time and research put into it?
A Common List of Advantages and Disadvantages
Advantages of 2 Stroke Engines:
- Two-stroke engines do not have valves, simplifying their construction.
- Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost.
- Two-stroke engines are lighter, and cost less to manufacture.
- Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.
Disadvantages of 2 Stroke Engines:
- Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
- Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.
- Two-stroke engines produce more pollution.
From:
- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.
- Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
- Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.
- Two-stroke engines produce more pollution.
From:
- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.
Clearing the Air
Most of what is written on advantages and disadvantages of 2 strokes Vs 4 strokes is not actually correct.
Take for example the lubrication issue of 2 stroke engines, sure small chainsaw engines may have the oil mixed with the fuel but this is not a direct result of the engine being a 2 stroke, this is just a result of someone designing a very simple engine. look at any large Caterpillar, or Detroit 2 stroke they have conventional oil sumps, oil pumps and full pressure fed lubrication systems and they are 2 stroke!
Finally, the arguments of simplicity, weight, power to weight, and cost of manufacturing are not a function as such of 2 stroke versus 4 stroke engines. The mistake of most of these commentaries is that they are comparing a simple chainsaw 2 stroke engine with a complex 4 stroke engine from a automobile - not a very fair comparision.
As far as the exhaust emissions of 2 strokes - check out the Surrich/Orbital 2 stroke design that Mercury outboards are using - this is as clean burning as any 4 stroke.
The ONLY correct comparison of 2 strokes with 4 strokes is that a 2 stroke can (in theory) produce twice the power of a 4 stroke for the same sized engine and the same revs.
The Better One
At the end of the day the winner is probably going to be the one that has had more money and technology spent on it-The four stroke engine. In these days of quick and cheap international production schedules you can't take it for granted that the 4 stroke will be better. So for your particular application, line up the options and make a decision based on what's available, not based on lists that miss the key points of difference..
