Engines 101 - Basic Theory and Trouble Shooting


This post is intended as a bit of a primer for those new to working on engines. It will contain some basic theory as well as initial trouble shooting ideas. The idea here isn't to answer all the questions you may have, but at least get you familiar with how your engine works, why it works, and maybe lead you toward some more questions you may want to ask. No big math equations for this post and no complicated details. This is just what your engine has in common with all the others out there.

Four Stroke Engines
Four stroke engines are named because the piston completes four strokes (down, up, down, and then up again) for every complete cycle of operation. These strokes are named, "Intake", "Compression", "Power", and "Exhaust", respectively. If you prefer, the version with a bit of innuendo is Suck, Squeeze, Bang, Blow.

Each engine will have one crankshaft which transmits the power from your engine, through a transmission, to the rear wheel. Attached to the crankshaft via connecting rods will be at least one piston and there will be as many rods as there are pistons, one for each. Each piston resides in a cylinder, which contains the combustion (burning of the fuel and air). Pistons will travel up and down within the cylinder as the crankshaft spins, but it should be noted that up and down are relative terms. It would be more accurate to say that the pistons travel away from and then back toward the crankshaft. Access to the inside of the cylinder for the fresh fuel air is controlled by the intake valve(s) and spent exhaust exits from the cylinder when the exhaust valve opens. In all of our engines, these valves are called poppet valves, which generally look like this:

The valves are opened and closed using a variety of different methods depending on the engine configuration with newer engines usually making use of overhead camshafts and older engines using an underhead camshaft and pushrods. In the case of overhead cams, there will be lobes that depress (read: open) the valves themselves or will make use of a lever called a rocker arm. Pushrod engines will almost always use a rocker arm. Each camshaft, whether under or overhead will be mechanically connected to the crankshaft to spin at half of the speed. So when your engine is spinning at 9000 RPM, your camshaft will be spinning at 4500 RPM. This is directly related to having four strokes per cycle of operation. If you were running a six stroke engine (yes, there is such a thing), then the camshaft would spin at one third the speed, allowing the piston to travel down three times, and up three times for every revolution of the cam.

Intake Stroke
During the intake stroke, fresh air and fuel enter the cylinder through the intake port and then past the intake valve (or valves). Newer, and more performance-oriented, engines will often have more than one intake valve but will generally have only a single port leading to those multiple valves. Two valves are most common, with three showing up every now and again. The reason the fresh fuel and air enter the cylinder is because as the piston descends, the available volume within the cylinder increases. Increased volume leads to low pressure and because nature doesn't like low pressure, fresh fuel and air are pushed, by gravity, into the cylinder.

Compression Stroke
After the piston descends and starts its ascent, the intake valve closes, effectively sealing the cylinder. Now, as the piston rises the fuel and air mixture is compressed. This compression step is necessary to the running of your engine as uncompressed fuel burns too slowly to turn the engine. The more compressed the fuel and air mixture, the faster it burns.

Power Stroke
The power stroke begins with a spark from the spark plug. This ignites the compressed fuel/air mix and the heat from the combustion causes the compressed mixture to build a significant amount of pressure. It is this pressure which pushes the piston back downward (which, of course, causes the crankshaft to turn and creates all the power used by your bike). During the majority of this stroke, the valve remain closed, allowing the pressure to continue to push against the piston.

Exhaust Stroke
As the piston is pushed back down to the bottom of its travel, the exhaust valve begins to open. This allows the burned exhaust gases to begin exiting the cylinder. At first, the pressure from the combustion event is what carries the gases out of the cylinder as the cylinder pressures will still be much higher than those in the exhaust pipe. As the piston begins to rise toward the top, its travel helps with this process. The exhaust stroke is necessary to expel unwanted gases from the cylinder as these gases do not burn as well as fresh fuel and air, and any unexpelled exhaust gases will be taking up space that could otherwise be filled with fresh fuel and air, ready for the next combustion event.

All four of the above events can be seen in action, courtesy of this graphic from Wikipedia:
Click Here

Two Stroke Engines
Two stroke engines differ from four stroke in several key way. First, as the name implies, the piston only complete two strokes of the cylinder for each combustion event. The engine is still required to perform all of the same functions (intake, compression, power, and exhaust), but there isn't a clear delineation between these events. Each time the piston descends, it is a power stroke. But it's also when the exhaust gas has a chance to escape from the cylinder and when fresh fuel/air mix is brought in. Only the compression stroke is really comparable to the four stroke engine.

Access to the inside of the cylinder isn't usually via poppet valves, like on a four stroke. Also unlike a four stroke engine, the intake port on a two stroke is usually fed from the inside of the crank case rather than directly from the carbs. The most common method for our old two stroke engines will be via piston-controlled ports cut into the sides of the cylinder. As the piston descends, it will first uncover the cutaway for the exhaust port. The exhaust gases will begin to rush out of the cylinder. As the piston continues to descend, it will next uncover the intake port. The intake mixture now enters the cylinder due to combination of pressure from the descending piston (the descending piston pressurized the crank case, which contains the fuel/air mix and that increased pressure forced the mix into the cylinder) and the exiting exhaust gases will tend to "pull" the intake mixture in as they leave (think of how your ears sometimes feel when you close the last car window at high speed, a slight vacuum can often be felt and this is the same effect).

Because two stroke engines have a combustion event every time the piston descends (instead of every other time) they are usually more powerful than four stroke engines of the same size. Also, due to their lack of poppet valves, they also tend to take up less space and are easier to work on. On the down side, because of the lack of distinct intake and exhaust strokes, there is often a fairly large loss in efficiency, meaning they consume a lot more gasoline per mile. Generally speaking, a two stroke engine will have a similar power output to a four stroke engine 1.5 times its size and will consume as much fuel as a four stroke engine twice its size.

Also, unlike four stroke engines, two stroke engines will almost always have their oil mixed in with their fuel. This mixing can occur when the bike is running via a mechanical injection system which feeds from a separate oil tank, or the oil can be mixed directly with the fuel every time the gas tank is filled. The latter option is called "pre-mix". The need to include oil into the fuel is because the crankcases are pressurized (by the down stroke of the piston) in order to help push fresh fuel/air mix into the cylinder. This method of lubrication is not as complete as in a four stroke engine and usually results in a shorter time between engine rebuilds when compared to their four stroke cousins.

A two stroke engine animation can be seen here, again courtesy of Wikipedia:
Click Here

Gasoline Engine Characteristics
Despite the different operating characteristics, all gasoline powered engines REQUIRE the same three things in order to work:
1.) Fresh fuel and air mixed in the correct quantities. Too much fuel or too much air and the engine will run poorly or not at all.
2.) Compression. Without compression, the fuel and air burns at too slow of a rate and cannot produce enough power to push against the pistons. A 4:1 ratio is often considered to be the absolute minimum for a running engine (but this engine will be extremely sluggish and not very powerful). Two stroke engines will usually run in the 6:1 to 8:1 range. Four stroke engines will almost always be 9:1 or better, with some topping the 12:1 mark.
3.) A spark introduced at the correct time. The spark is what ignites the fuel/air mix. If you ignite it at the wrong time you're either losing compression because the piston is already descending or you're fighting against the piston still rising. In either case, your engine will not run well or at all.

Basic Trouble Shooting
Hopefully, the last section will make it clear what must be done in order to have an engine run well. Compression and spark are generally quick and easy to test and so these things should be checked for ANY engine problem that occurs. Make sure you have a timing light and a compression tester. Each of these tools can be had for around $30 and will almost certainly be necessary at some point.

After you have ruled out compression and timing, then it's time to look at fueling. If the engine won't start at all, pull the spark plugs and take a look. If they are wet, then your problem is too much fuel or not enough air. Ensure your carbs are working properly and adjust accordingly. Try turning in the idle screw a few turns to open up the throttle. That may help. If the plugs are dry this can be not enough fuel or not enough air. Yes, I know, I said that not enough air is a symptom when the plugs were too wet. This time the plugs will be dry because not enough air is coming through the carbs in order to pull the fuel along with them (more on carb theory is available in this thread: http://www.dotheton.com/forum/index.php?topic=16745.0). Again, try opening up the idle screw a little and checking the carb sync. The other possible reason for dry spark plugs is a clogged carburetor. If you haven't cleaned the carbs in a couple of years (or even since you got the bike), now may be a good time.

Remember, if your engine isn't running, there WILL only be three reasons. Because spark (and spark timing) and compression are so easy to rule out, this should ALWAYS be done first.

DTT Bike Of The Month Gallery

DTT Light or Dark

Top Bottom