Tiny explosions occur inside the engine and that’s what makes it go. Squirt some gas into the engine at just the right time, light it off, and you’re ready to go. This is true enough but the best way to describe the combustion that takes place inside an engine is as a process of energy conversion. Convert energy from a form that is cheap, portable and easy to store, into a form that will move us down the road. That is what the combustion process is all about.
A few simple things are needed to make this energy conversion take place smoothly: spark, fuel, and compression. That’s it; nothing else is required, except for maybe the proper timing of these three things. All of these must happen at the right time, or at least close to the right time and the engine will run. In order to have compression, the intake and exhaust valves must be closed, and the piston must be moving in an upward direction within the cylinder. This is easy enough, what about spark and fuel?
The fuel is where the energy is. This energy is in a chemical form and in order to make this fuel propel the vehicle down the road it must be converted into heat energy. Over the years the fuel distribution mechanism has evolved, and in the process has grown more and more efficient. For decades the fuel going into the engine was blended with the intake air stream in the carburetor. These old, inefficient devices were last found on new cars in the early 90’s, since that time all cars sold have had some form of a fuel injection system. The carburetors use what’s called a venture to create a low pressure area that relies on atmospheric pressure to push the fuel into the intake manifold where it vaporizes as it gets sucked into the cylinders. This is very inaccurate and leads to poor fuel economy, dirty exhaust emissions, and less power output.
Fuel injection systems will either spray the fuel into the intake onto the back of the intake valves, or spray the fuel directly into the combustion chamber. This is very accurate and each individual cylinder will get the exact amount of fuel that it will need to make the most of the capacity of the cylinder and the amount of energy that is in the fuel.
When the air and fuel are mixed, and the mixture has entered the combustion chamber, both the intake valve and the exhaust valve will be closed and the piston will move be moving up in the cylinder. The piston moving up will compress the air fuel mixture and squeeze it into a very small space at a ratio of about 10:1. Squeezing the mixture like this concentrates the oxygen in the fuel and the heat in the cylinder. Both of these things help to make the combustion much more powerful and the consumption of the fuel much more thorough.
When the air/fuel mixture is fully compressed a spark will be fired to the spark plug where it jumps the air gap between the two electrodes of the plug. This spark introduces a small source of heat that lights the air/fuel mixture. When the air/fuel mixture burns it causes a tremendous increase in temperature, and when temperature goes up, pressure goes up. As this pressure wave propagates within the combustion chamber, the piston must be in the right position to take the brunt of this expanding force. The piston takes the force of this expansion and moves down in the cylinder exerting tremendous force on the connecting rod, which connects the piston to the crankshaft. The crankshaft turns the reciprocating motion of the piston to the rotational motion that goes to the wheels.
Much is happening within the engine and considering how fast all of this takes place, it’s a wonder that the engine runs as well as it does. Not only does it run well and produce gobs of power but it can do it for hours on end, day in and day out with very little trouble.
In the old days the spark would originate in the ignition coil which essentially works like an electrical transformer. The coil takes a small amount of voltage with a high amount of current, and produces a high amount of voltage with a small amount of amperage. This spark is produced in accordance with the physical position of the pistons in the cylinders. When a piston is on the compression stroke and nearing the top of the run, on older engines, the coil would fire a spark to the distributor which would then send the spark to the appropriate cylinder at the appropriate time. The distributor was mechanically timed to the crankshaft and the camshaft so that the rotor that was spinning in the distributor would be lined up with the spark plug in the cylinder that had a piston nearing the top of the cylinder.
On the most modern ignition systems found on today’s engines, each cylinder has its very own coil. No mechanical connection is needed between the engine and the coil controls. A computer looks at piston position via crankshaft and camshaft position sensors, and when the time is right it will fire each individual coil for each cylinder. This is extremely accurate and very efficient. This way of firing the spark requires fewer moving parts, and fewer parts in total. This system allows the computer complete timing control. Eliminating moving parts and turning all control over to the computer makes the engine more efficient.
In order to have good, strong combustion, the spark that lights off the air fuel mixture must be introduced at different times depending on how the engine is operating. The amount of time required for the air/fuel mixture to burn is usually about the same no matter how the engine is running. Combustion occurs very quickly, so much so that it seems like an explosion, but in reality it is a very controlled process. In order for the piston to be in the right position to accept the rapidly expanding air, the spark must be introduced at just the right time.
On nearly all engines the spark must hit the air gap of the spark plug before the piston is actually all the way at the top of the cylinder. When the engine is running at high RPM’s the spark must be introduced even sooner because it will take just as much time for the air/fuel mixture to burn. This early timing of the spark is referred to as timing advance. The faster the engine is running the more advance is needed. This timing must be precise in order to maximize power output and efficiency. This early introduction of the spark is measured in degrees of crank shaft rotation before the piston is at the very top of the cylinder. When the piston is at the top of the cylinder it is said to be at top-dead-center or TDC.
If the spark is introduced too late, then by the time the air/fuel mixture burns thoroughly, the piston will be so far past top-dead-center that the expansion of the air will not exert as much force on the top of the piston. If the spark is introduced too early then the combustion process will push on the piston when it is still in a position before TDC, this not only does not produce very much power but it can also be very damaging to the engine. The piston essentially slams into a rapidly burning and expanding air/fuel mixture. When this happens it is known as knocking or pinging. This knocking usually produces a sound deep in the engine that sounds like a rattle. This is a very bad thing, but usually only happens when something within the engine control systems is not working properly. Knocking can also be a problem if the fuel that is used in the combustion process has too low of an octane rating.
|The ideal position for the piston to accept the power of the combustion in this example is 23° after TDC. At 1200|
RPM the spark must fire at 18° BTDC and at 3600 RPM the spark must fire at 40° BTDC.
In order to make sure that spark timing stays exactly where it needs to be for the varying engine operation, a computer receives information from several different sensors. A cam sensor and a crank sensor look at piston position, and engine speed. A throttle position sensor looks at throttle position to see what the driver wants the engine to do. A coolant temp sensor looks at how hot or cold the engine is because this too has an effect. The computer can even look at a sensor called a knock sensor to see if the combustion process is happening to soon. This early combustion is the engine knocking that was explained above.
When the knock sensor detects engine knock, the circuits in the engine control computer that adjust ignition timing will back off the timing of the spark so that no engine damage will occur. This means that the spark will be fired at the spark plug closer to TDC. The ability of the computer to rapidly adjust this timing advance is a drastic improvement over the way timing advance used to work. On old engines, timing could never be advanced as much as would be considered ideal, because the mechanisms that controlled timing advance where crude mechanical devices that were slow to react and imprecise.
Burn It Good
Despite the fact that combustion controls are far more efficient then they used to be, the gasoline powered internal combustion engine is still only about 25% percent efficient on average. This is much better than the 15% efficiency that was common in the old days. Most likely the internal combustion engine will continue to become more and more efficient but the likelihood that it could become as efficient as an electric motor is not very high. Diesel engines are more efficient than gasoline engines but they still waste a tremendous amount of energy compared to electric motors. As long as the internal combustion engine keeps getting better and better, and the cost of electric cars stays high, we will keep on driving the cars that we know best.