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Two stroke vs four stroke dirt bikes
Transcript of Two stroke vs four stroke dirt bikes
Two stroke engines have a two stage process of compression, when the piston moves up and compresses the air and petrol/oil mixture, and combustion when the spark plug ignites the mixture forcing the piston down. The engine ignites the spark plug at the top of every revolution of the piston, igniting the petrol air mixture, forcing the piston back to the bottom opening the intake and exhaust valve. This is seen as a negative in the system as the intake valve and exhaust valve are open at the same time, some of the petrol and oil mixture goes straight out of the exhaust valve un burnt. This means that there is wasted petrol mix that is not being used and that the engine is more emission heavy.
Two stroke vs four stroke dirt bikes
Four stroke engines have a four stage firing order: an intake stroke is the first stroke down to allow the air and petrol mixture into the cylinder. The upwards stroke after that is the compression stroke that compresses the mixture. the second downward stroke is the combustion stroke where the mixture is ignited forcing the piston down. The final stroke is the exhaust stroke that releases the emissions on the final upward stroke. With the intake and the exhaust valves being open on different strokes, no unburnt petrol escapes through the exhaust valve making the four stroke more fuel efficient and having less of an impact on the environment.
Since the invention of the four stroke dirt bike, there has been a debate about which bike is better, the two stroke or the four stroke. It has given riders an option on what kind of bike they would like to ride, the unpredictable power delivery of the two stroke, or the predictable steadiness of the four stroke. The debate on which bike to choose as a first time rider is the biggest debate in the motocross world. The topics that will be covered are:
The difference in firing order (the two strokes vs four strokes)
The difference in weight of engine
How the bikes ride
To gain the information in this presentation, a number of primary and secondary sources such as online, websites and surveys.
through this I will come to the conclusion that both bikes have their advantages and disadvantages.
The firing order of the two bikes are very similar. They both do all the same thing just in different ways. The four stroke delivers power on every second stroke of the piston, whereas the two stroke delivers power on every stroke of the piston. This makes the two stroke twice as powerful as a four stroke of the same engine size. But this also makes the power delivery in a two stroke very unpredictable and in some cases harder to control. The four stroke is more tamed in its power delivery making it more suitable for first time riders.
Two stroke engines have a higher fuel consumption as the intake and exhaust valve are both open at the same time, releasing some unburnt petrol.
Two stroke is the lighter bike because of the simplicity in the engine. They have less moving parts than a four stroke due to the piston working as a, piston, the compressor, an intake and an exhuast valve. This means that the engine is smaller and lighter than a four stroke which means it can fit in a smaller frame, making the bike even lighter.
Four stroke engines are heavier due to having a much larger cylinder head to fit the cam/timing chain, camshafts and valve rockers. this makes the four stroke engines have a lower power to weight ratio.
Yamaha YZ250F four stroke
How They Ride
Two strokes can be a little unpredictable with their power delivery compared to the four stroke. This is due to the engine delivering power on every stroke of the piston.
Two strokes give out ten times the horsepower and torque. In the case of he Yamaha YZ250 two stroke and the Yamaha YZ250F four stroke, the YZ250 gives out 46 horsepower and 30.6 ft-pounds of torque compared to the YZ250F which gives out 36 horsepower and 20.1ft-pounds or torque. This means the YZ250 is a lot faster out of the gate.
For the YZ250, the powerband is very broad and usable. The explosive hit of the powerband can be unpredictable and often just makes the rear wheel spin or cause the front wheel to come up off the ground. This is because the powerband in at two stroke is a sudden surge of power that kicks in when the bike gets to a certain revs per minute (RPM).
In a straight line race, the YZ250 will win every time. The two stroke has a higher top speed and is faster out the gate, but a motocross race is never a straight run. In the soft sand, steep hills and big whoops (bumpy section of track) the two stroke excels over the four stroke.
Comparing Yamaha YZ250 Two stroke to Yamaha YZ250F four stroke
The four stroke is a lot easier to handle due to the engine delivering power on every second stroke of the piston. This makes the power delivery predictable and steady which is perfect for a first time rider or someone who just wants to ride around casually with no sudden bursts of power.
A four stroke 250 may not have anywhere near as much horsepower or torque as a two stroke of the equivalent cubic centimeters (cc) but the AMA rules state that a 250 two stroke must ride in the 450 four stroke class, giving the fourstroke the upper hand.
The powerband in a four stroke is completely different to a two stroke. With a four stroke you gradually gain speed by accelerating with no sudden burst of power at any point.
In slick conditions with limited traction is where the four stroke has the upper hand on the two stroke stroke.
Yamaha YZ250 Two stroke
Two strokes are the cheaper and easier bike to maintain. They have little to no moving parts compared to the four stroke that need to be adjusted, rebuilt or that can be broken. The biggest part of the maintenance and the most expensive part is the top end. This is done to optimise the compression after the piston and the piston rings have worn out. This process is a lot easier and cheaper in a two stroke as the top end only consists of the piston and the piston rings, making them the only part that needs a rebuild. A top end rebuild should be done roughly once every year or two to optimise compression and to stop the engine blowing up.
The other big part of two stroke maintenance is making sure that the oil/petrol mixture is the right consistency, if it is not mixed to the correct ratio it can destroy the engine. Every bike has a recommended ratio from the company that made the bike, but this ratio is for a professional rider that has the bike at high revs for races. But if the bike is ridden at mostly low to medium revs, the oil is wasted as the engine doesn't need that extra lubrication. The buildup of oil can foul up parts of the engine like the carburetor and the spark plug. But this is a better option to not putting enough oil in the mixture and burning out the engine. The top end may need to be rebuilt more often than a four stroke but it is still a lot cheaper than the frequent servicing of the four stroke.
Four stroke dirt bikes require frequent servicing to make sure all of the extra moving parts in the engine are still moving properly. These services are very expensive if they can not be done at home due to being ill equipped or unknowledgable and must be booked in to a professional. Four strokes can be maintained at home to cut down on the expensive professional services, but it is very hard as there is a lot of extra parts that have to be taken into consideration. Unlike the two stroke which is very easily maintained at home and it is a simpler engine.
There is no need to mix oil in with the petrol as the crankcase in a four stroke is a reservoir for the oil, meaning the crankcase is filled separately. Oil must be mixed in the two stroke as the crankcase is used to suck in the air/petrol/oil mixture. Without the extra oil having to be added in, some parts of the bike stay cleaner such as the spark plug and the carburetor as no excess oil is being released.
A two stroke dirt bike would be suited for a more experienced rider that knows how to ride. The unexpected kick of power from the powerband is exciting for someone who knows how to handle it, but for a first time rider it can be scary and make them lose control of the bike. This does not mean a first time rider can not get a two stroke, they would just have to be more confident in their riding.
Two strokes have an advantage over four strokes on maintenance. They have a lot less moving parts that can break and need to be replaced, with most of the maintenance being able to be done at home if the person is properly equipped and well trained. The only downsides of maintaining a two stroke is having to mix oil in with the petrol at the correct ratio and having to constantly rebuild the top end. This is still cheaper than all the servicing that a four strokes require.
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Both bikes have their advantages and their disadvantages. The decision of two stroke vs four stroke basically come down to personal preferences and how the person likes to ride.
So if the bike will be getting in to races, a four stroke would be the bike to chose due to the AMA rules. But if it will be a weekend bike that will be thrashed around in the bush then a two stroke is the way to go. This is not to say that that a two stroke can not be used in racing and a four stroke can not be a great bike to ride out in the bush though.
So, the biggest debate in the motorbike world about whether a two stroke or a four stroke dirt bike is the better bike all comes down to rider preference and how a person wants to ride the bike. Either way they are both great bikes to ride.
A four stroke bike is more suited for a first time rider that likes the predictable, steady increase of power. This makes the bike easier to handle, thus less likely for a first time rider to fall off and not want to ride again.
The four strokes give out less emissions than the two strokes,this is an advantage from an environmental point of view.
They are also the quieter bike which is an advantage if it will be started in a residential area.
Most professional motocross riders actually ride four stroke bikes, but this is mainly due to the AMA rules stating that a two stroke dirt bike has to ride in a higher class, such as a 250cc two stroke having to ride in the 450cc four stroke class.
(2 stroke & 4 stroke cycle engine, 2014)
2 stroke & 4 stroke cycle engine. (2014). [video] MonkeySee.