Timing, they say, is everything, and that’s particularly true with engines. Understanding how and when the individual events happen, and why they happen when they do will help you understand why they have to be set up as they are, and that always makes it easier to figure out what’s going on when problems arise.
Let’s start by going over the basic way the engine works, and what events need to occur and when. First, we’ll look at the simple basics, then at the reason things happen exactly when they do. To simplify the discussion, and to make it more closely relevant to dirt bikes, we are only going to discuss single cylinder engines during this explanation.
The term, “four-stroke cycle” means that the engine needs to move the piston up or down the bore 4 times to complete all the functions that go into producing power from gasoline. Because the piston is connected to the crank via the connecting rod, each “stroke” takes a half revolution of the crank, or two full revolutions for the four necessary strokes.
The Four-Stroke Cycle
A lot of folks have seen this simplified version, but let's review. The cycle starts with the Intake Stroke, near top dead center (TDC), where the piston is at its highest possible position, with the intake valve opening and the piston moving down the bore toward bottom dead center (BDC). This creates a void above the piston that is filled by air from outside rushing in through the intake port to fill it, and that air carries with it the fuel added by either the carburetor or a fuel injection system.
The rotating crank then begins to move the piston up the bore and the intake valve closes, trapping the fuel and air in the cylinder. As the piston continues upward, the air/fuel mix is compressed, which heats it and increases the amount of force with which it will expand when ignited. This is the compression stroke.
The power stroke begins near the top of that second stroke, when ignition takes place, starting the fire. The crank rotates past TDC as the burning fuel begins to expand, and the combustion force pushes the piston down the bore, creating the rotating force on the crank that drives the whole works.
As the piston nears BDC again, the exhaust valve opens, and the piston is run up the bore to pump the spent gasses out through the exhaust port to complete the cycle with the exhaust stroke.
Notice something here: The engine’s crankshaft rotated twice to produce four trips up and down the cylinder for the piston, but each valve only opened and closed once during that time. To make this work, the camshafts have to turn at one half the engine speed, so the chain and sprockets, or gears, or toothed belt and sprockets used to drive them are set up at a 2:1 ratio.
Ignition also has to happen right on time, so each ignition system, whether simple ‘50’s style points, or the most sophisticated electronic, has to have something to signal when that is. Traditionally, this signaling trigger has been attached to the camshaft so that the spark occurred only once every other revolution, but engineers seeking to simplify the design of single cylinder dirt bikes found no reason that there could not be a spark on every revolution, so the trigger sensor was mounted at the crankshaft instead. That means there is a spark on every revolution, instead of only once per each two-revolution cycle of the engine. The second spark happens at the end of the exhaust stroke, so there’s nothing present in the cylinder that would burn. It also makes setting up the timing during assembly somewhat simpler by eliminating what used to be a common mechanic’s mistake of picking the wrong top dead center position.
Getting Ahead of Things (The Engine is Dynamic)
Simplified explanations of the cycle like the one we started with here always show the valves opening and closing right at TDC and BDC, but if you watch the piston position as you turn an engine over by hand to watch the valve gear operate, you will notice that the valves don’t open and close at the exact top and bottom of their respective strokes. That’s because the engine is a dynamic system, which means it’s something that moves, and it does so at a pretty high speed. Most MX 450’s make peak power at around 9000 RPM, which means they make two full revolutions and complete an operating cycle in about 13 milliseconds at that speed. The crank spins continuously, but the intake, exhaust, and combustion all stop and start again while that’s going on. That means that all of these events actually have only a certain amount of time in which to occur, so they have to be started in advance so that they happen on time. Again, we’ll look at the intake stroke first.
With the crankshaft spinning along at a few thousand revolutions per minute, if we were to wait until top dead center to open the intake valve, the piston will travel well down in the bore by the time the valve is open wide enough to let much air into the cylinder, so the intake valve begins to open around 20 degrees or more before top dead center (BTDC). This does a couple of things. For one, the exhaust stroke is just ending, and the inertia of the spent gasses leaving the cylinder creates a bit of a vacuum that helps get the intake air moving in. There’s a little bit of built up pressure right behind the intake valve as a result of the intake valve having been slammed shut on a moving column of air at the end of the previous intake stroke, and that helps, too. But mainly, we want the intake valve to have time to be open nice and wide as the piston moves through the fastest part of its down stroke so we can get the cylinder as full as possible. On top of that, we’re going to keep the intake valve open until well after bottom dead center (ABDC) to take advantage of the inertia of the incoming air.
Which brings us to the compression stroke. The piston is now rising and pushing against the load of incoming air, stalling the flow into the cylinder, so the intake valve closes as this balance is struck, about 130 degrees BTDC. With both valves now closed, the piston compresses the air and fuel mix to less than 1/10th its original volume to heat it up and to increase the force with which it expands as it burns. This compression will continue until TDC, but the ignition has to happen well before that in order to extract the maximum power from the burning of fuel.
The Power stroke, then, is initiated before the piston actually starts down. This “spark advance” allows the burning gasoline time to start at one small point near the spark plug and spread across the combustion chamber to the point where it becomes confined by the piston and must push it down out of the way. That’s where the power comes from. If the spark occurs too late (is “retarded”), the piston will outrun the fuel burn and not much pressure will be applied. On the other hand, if it happens too early (“advanced”) then too much pressure will be created while the piston can’t get out of the way fast enough, which leads to damage from detonation and the like. The faster the engine turns, the more advance the ignition needs to keep up, so modern systems advance the timing as the RPM increases.
At about 120-130 degrees ATDC, the energy from the fuel burn is so low that it really isn’t putting a lot of force on the piston any more, and the leverage that the piston has on the crank is getting pretty low, so the exhaust valve starts open before reaching BDC. The pressure that remains from the burn starts the gasses flowing outward, boosted by the piston as it rises and pumps the bore clear. The exhaust valve remains open past TDC to utilize gas inertia and help restart the intake airflow for the next cycle.
Am I 180 Out?
People ask this a lot when they have trouble getting an engine running after they’ve set the cam timing up, or when they bring the piston up to Top Dead Center and find both valves open. This is the common mistake we mentioned earlier, and it's one of the things that's more easily understood when you have a good grasp of the complete cycle. It’s more of a car thing, but if you have an old classic four-stroke from the ‘70’s or before that uses cam driven breaker points, it’s sometimes possible. These days, the answer is usually, “no.” The old way of connecting the ignition to the engine mechanically, that of using a distributor or some other device driven at half speed by the cam, allows a mistake in assembly to be made. A mechanic could position the engine at TDC, and if not careful to check, he could position the ignition trigger to fire during the exhaust stroke instead of the compression stroke. This was referred to as being “180 degrees out” because the distributor or point plate was 180 degrees away from the correct position on the camshaft because of this. Actually, going by the crank, the ignition timing was 360 degrees out.
But with the ignition trigger located on the crankshaft instead, as is the case with virtually all modern single cylinder dirt bike 4 strokes, that’s not possible. Without the cams connected to the crank, one TDC is exactly like another; the rod’s at the top, and the spark signal is given as the crank gets there, every time. So, the only thing that determines which stroke is which is the camshaft(s), and how they are positioned by the assembler. That’s why the service manuals for such engines make no mention of checking for which of the two different TDC’s is used. In operation, there is a second, "wasted" spark that happens near the end of the exhaust stroke.
What About Automatic Decompression?
This is another area where really understanding the four-stroke cycle helps clear things up. It's extremely common to hear people tell someone with a modern four-stroke single to "find TDC" before starting, but that's wrong. When you turn the engine over slowly, you find it rotates fairly easily until you come to a "hard spot". Without auto decompression, the hard spot is the point at which the intake valve closes to begin the compression stroke. That looks like the picture below, "Non AD".
From this point, you would need to force the engine to compress about 80% of it's full stroke length worth of air, and that can be nearly impossible with the high compression ratios used these days. What automatic decompression does is use a speed sensitive mechanical system to lift the exhaust valve off its seat at very low speeds (slower than the engine will idle at) until the engine gets a lot closer to TDC, but not past it, so that when kicked over from this position (or spun through it by a starter motor) there will still be enough compression to start, and both valves will be closed as the spark happens and the end passes top dead center. That looks like the "Auto Decomp" picture above. You can see that there will be a lot less effort needed to compress the air/fuel charge from here than from the normal, non auto decompression setup. This, by the way, is where you want to be if you have an older manual decompression engine. If you go past TDC instead of stopping just prior to it, you would have to kick the engine through nearly two full revolutions to get back to the compression stroke again, and it would still be at full strength.
Once you have the whole picture set in your mind, you'll make fewer assembly mistakes, and you'll be able to catch on to problems more quickly. A crusty old mechanic told me a long time ago, "The best way to figure out why something works wrong is to know how it's works when it works right".
Edited by grayracer513