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Today’s 4 stroke dirt bike engines are leap years better than they were 10 years ago. What has changed? Technological advancements have seen the weight decrease, the power increase and the all round performance of the 4 stroke engine become so much more efficient.
The downside of this is today’s high performance thumpers are far more costly to maintain than they used to be. Though they generally don’t require as much regular maintenance as a two stroke – but when they do there are more parts to replace, therefore costing you more.
The 4 stroke dirt bike engine consists of a piston and barrel, and the cylinder block which holds the valve train. The number of valves vary with different bikes – The ’08 YZ450F has 5, while the ’07 CRF450 has 4.
Just like the 2 stroke, the piston moves up and down in the cylinder block powered by explosions from combusting fuel/air mixtures triggered by the spark plug.
But, unlike the 2 stroke, the thumper engine only gets one ignition, or explosion for every four strokes of the piston.
YZ450F Piston & Crankshaft
The four stroke engine has four unique strokes or movements that each perform a specific task.
- The first movement is the intake stroke. As the piston moves from the top of the cylinder block to the bottom, the intake valve/s open to allow a mixture of fuel and air to be drawn in to the cylinder.
- The second movement is the compression stroke. All valves are closed as the piston rises, causing the fuel mix to be compressed.
- As the piston reaches the top of its stroke, the compressed fuel mix is ignited by the spark plug, causing an explosion which forces the piston back down. This is the combustion stroke.
- Now the piston begins to rise again. The exhaust valve opens up and releases the emissions from within the cylinder, out through the exhaust pipe. This is the exhaust stroke.
This whole process then repeats…
The powerful YZ450F
The 4 stroke dirt bike engine, unlike a 2 stroke does not have fuel inside the crank case. Therefore, you are able to fill the crankcase (where the crankshaft bearings and moving parts of the piston rod are) with a heavy duty oil to lubricate them.
So, that is why you do not need to mix oil into the fuel with a four stroke engine. No burning oil – less damage to the environment.
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