How does a Ninja 250 engine work?

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Our engines, and those in most modern vehicles, have four strokes: intake, compression, power, and exhaust.


Intake: The intake valves are open on the intake stroke. The piston goes down, sucking in a mixture of fuel and air from the carburetors.

Compression: No valves are open on the compression stroke. The piston goes up, compressing the air/fuel mixture.

Power: No valves are open on the power stroke. The spark plug fires, igniting the now-compressed fuel/air mixture. This sends the piston hurtling downward.

Exhaust: The exhaust valves are open on the exhaust stroke. The piston goes up, forcing burnt fuel/air out the exhaust.

The engine will run if:

  • The spark is strong enough.
  • The fuel/air mixture is correct.
  • The timing of the valves opening/closing is correct.
  • The timing of the spark ignition is correct.

The timing of the valves is fixed by the cam chain and its relation to the crankshaft and camshaft. The timing of the spark is slightly variable and is timed with electronics, based on the position of a magnet/coil sensor on the alternator flywheel (which is on the left hand side of the crank shaft).

The job of the carburetor is to meter the air:fuel ratio. It needs to stay close to 14 parts air for every one part gasoline for the mixture to be explosive when compressed.

Our carburetors are quite complicated as far as carburetors go, as they have to work over a wide range of engine rpm, temperatures, and operator input. They are a constant-velocity type, which, to grossly oversimplify, means they open up more the harder the engine sucks on them during the intake stroke. How hard the engine sucks on the intake stroke is directly related to how much power it made on the last power stroke. This is all tempered by the position of the carb's mechanical butterflies, controlled by the throttle twist grip.

When you decelerate, the sudden closing of the throttle butterflies can cause an overly lean (too much air, not enough gas) mixture, which won't ignite. This can cause the bike to lightly backfire because of unburnt fuel reaching the exhaust; it sounds kind of like a light burble. In an extreme circumstance, it might even cause the bike to stall.

To make up for this, there is a coasting enrichener plumbed into our carbs. This is the snail-shell-kinda-thing on the left one. The vacuum hoses operate its diaphragm and can "feel" when there is a sudden change of the throttle butterflies slamming shut; when this happens they let a wee bit of extra fuel into the mixture to prevent these lean-miss backfires.

The throttle butterflies cannot be allowed to close 100%, otherwise the bike will stall, since no fuel/air at all will reach the engine during the intake stroke. This is what the idle adjuster does: It's a throttle stop that prevents the throttle spring from completely closing the butterflies.

Making all this happen is actually a lot easier than it sounds. If you keep up with the maintenance on your machine and make sure all the individual parts of the system are working correctly, the bike will put all the processes together for you and get you where you want to go.

For more information: This page by Animated Engines also shows and explains how the four cycles of a 4-stroke engine work.