While engine performance in a modern car or light truck involves some very complex control systems, the basics haven't changed since the inception of the internal combustion engine. Every modern engine, no matter the level of sophistication of its control system, must have good fuel delivery, sufficient spark, proper compression, and generally unrestricted exhaust.

In really simple terms, the fuel has to get in there, get compressed, explode, and get out efficiently if the engine is to run properly. Engine performance and drive-ability problems occur when one or more of those steps is interrupted, or pushed out of specs.

Of the four, compression and exhaust are primarily hardware dependent, while fuel delivery and spark depend not only on the hardware components but on the engine control system.

Fuel Delivery

While some "alternative fuel" vehicles are under development, or operating on a test or prototype basis around the country, virtually all of the vehicles your customers are likely to work on are conventional internal combustion engines (either spark-fired or compression-fired) that run on gasoline or diesel fuel.

Since most cars and light trucks in the US fleet are gasoline-fueled, we'll concentrate on those here, thought much of the basic technology applies to both gasoline and diesel engines.

In order for the fuel to combust properly, it has to be mixed with air and the fuel to air ratio must be kept within a fairly small range if the engine is to run smoothly and efficiently. Fuel is moved from the tank to the fuel delivery system by way of a fuel pump that is typically locat3d in the fuel line just outside of the fuel tank, or submerged within the tank on most late-model light vehicles.

In older models the fuel pump was mechanically operated, and located in the engine compartment. In any case, the pump draws fuel from the tank and delivers it under pressure to the carburetor (older models) or fuel injection system (new models). The air enters through a duct that includes a plenum, or chamber, that houses a filter designed to remove dust and other particles. In a carbureted vehicle, the air filter housing sits directly over top of the carburetor inlet, while in fuel injected models, the air filter is often housed nearer the air duct's opening.

The carburetor or fuel injection system atomizes the fuel and mixes it with the incoming air to provide the proper fuel:air ratio for any given engine operating condition. During acceleration or high speed operation, the fuel/air charge per cylinder is increased; during deceleration, or low speed operation, the fuel/air charge is decreased.

Compression

Once the fuel/air charge is in the cylinder, the rising piston pushed the fuel/air mixture into a very small space at the top of the cylinder, compressing it so that it will explode and burn with its full power potential. Thin metal compression rings ride in grooves on the outside circumference of the piston to keep the fuel/air mixture from escaping into the crankcase through the annular clearance space between the piston and the cylinder wall.

The head gasket also prevents the compressed fuel/air charge (as well as the burned exhaust gas) from escaping into the cooling water passages or into adjacent cylinders.

The compression ration of a typical engine is a constant, based on the physical dimensions and geometry of the main crankshaft, the piston connecting rod, and the piston itself. It generally falls in a range of about 7:1 to 8.5:1, tough high performance street engines can have compression rations as high as 10:5:1, 11:1, or even 12:1. The higher the compression ratio, though, the higher the octane fuel required to prevent engine knock.

Ignition

To reap the benefits of the explosion of the fuel/air mixture, we have to provide a way to ignite it. In a gasoline engine, that's accomplished by the spark plugs.

For the spark plug to do its job right, the spark has to provide sufficient energy, at the right time, and at the right place to provide complete combustion of the compressed fuel/air charge.

Being in the right place is determined by the engine layout, including the location of the spark plug hole and the geometry of the combustion chamber, as well as the length of the plug and its electrode configuration.

The spark's timing is controlled by that engine control module in newer vehicles, or by the distributor in older models. In either case, the spark's timing must be adjusted continuously during operation. More specifically, the spark's timing must be advanced, relative to the position of the piston, as engine speed increases.

Why? Because the combustion chamber, which s the space remaining in the top of the cylinder when the piston is as high as it will go in the cylinder (i.e., at top dead center, or TDC)--is always the same. It has the same volume and geometry, regardless of engine speed.

The spark also takes approximately the same amount of time to propagate through the entire charge, regardless of engine speed. To get the most power out of the engine, though, the full fuel charge should be exploding and expanding just at the point when the piston is at TDC and can be driven downward by the expanding gases.

Since the time it takes for the spark to ignite the entire mixture is roughly constant, it has to start traveling through the mixture earlier when the piston is moving very fast, in order for the entire charge to be ignited at DC. Hence the need for timing advance.

Exhaust

Once the fuel/air mixture has been exploded and its power absorbed by the mechanical motion of the piston, the spent gasses must be purged from the cylinder in preparation for the next cycle. To keep vehicle emissions within acceptable limits, the exhaust gas must also be cleaned up on its way out of the vehicle, and the sounds of several thousand explosions per minute must also be silenced.

In the exhaust system, the clean-up job is provided in most newer vehicles by a catalytic converter, while a muffler or muffler/resonator combination performs the silencing function.

An oxygen sensor in the exhaust system also determines, continuously, whether the engine is running too rich, too lean, or just right and allows the control system to adjust fuel delivery accordingly.

Performance Concerns

As usual, understanding potential failures, and knowing how to fix them begins with knowing how everything works. Once you know that much, you can begin to surmise what components might contribute to a certain type of system behavior, and which can be eliminated out of hand.

Since we've already noted that good engine performance depends on the presence of a fuel/air charge, properly timed spark, good compression, and efficient exhaust, we know that bad performance must indicated the lack of one or more of these crucial ingredients.

Diagnosing the problem requires a systematic approach to checking each contributing factor, until we find the one or more that done meet specs.

For instance, if the engine will turn over consistently and steadily, but won't star, or if it starts, but runs poorly, we can usually eliminate the battery as a problem.

That leaves us with a possible lack of fuel, a lack of spark, a lack of compression, or a severely restricted (blocked) exhaust. To find the culprit, we have to check each, elimination possibilities as we go.

While it may seem embarrassingly simple, the first question to ask a customer who says his or her engine will turn over fine, but won't start, is what the gas gauge reads. While this may sound like a dumb question, it's a lot smarter asked up front than after a couple of hours of diagnostic work, especially if it gets that blank, deer-caught-in-the-headlights kind of stare from the customer. If there's fuel in the tank, then the diagnosis can proceed.

The most straightforward of our troubleshooting steps is usually the compression check. A compression gauge can be used on each cylinder, and any cylinder with low compression can be identified easily. That does give you the cause, but does narrow the search. Low compression can be the result of bad compression rings, valves that don'ts seal properly, a blown head gasket, or a cracked cylinder wall or head. Troububleshooting tools and techniques exist to identify each of these faults.

Exhaust problems, and general combustion problems can also be identified often by analyzing the exhaust gas chemistry. A number of sophisticated infrared exhaust gas analyzer exist for this purpose.

Hydrocarbon (HC) levels indicated the amount of unburned fuel in the exhaust. Carbon monoxide levels reflect the fuel/air mixture. High CO levels indicate a rich fuel/air mixture; low CO levels indicate a lean mixture. Anything that makes the mixture richer, like leaking injectors, leaking carburetor power valves, or a dirty air filter, will raise CO levels, while air leaks will lower CO levels. A very high hydrocarbon (HC) level indicates a misfire.

In newer engines, a bad oxygen sensor can also be the cause of too-rich or too-lean fuel/air mixtures.

An exhaust gas analyzer can even be used to check for the presence of exhaust gas coming form the radiator filler neck (which would indicate a blown head gasket or cracked block or head).

Lack of a properly timed spark of sufficient power can be a more complex diagnosis, because so many factors can contribute to such a condition--often intermittently.

Generally, ignition systems suffer from the same problems that afflict any electrical circuit; that is, open circuits, short circuits, shorts to ground, and abnormally high resistance.

The most basic reason for spark problems is a fouled or worn spark plug electrode, cracked insulator, or bad plug wire. The electrode condition can be checked by a visual inspection; testing the other possibilities requires basic electrical troubleshooting equipment.

The ignition circuit can have several design variations. In the traditional circuit, a coil provides sufficient voltage to cause a spark, and a distributor with rotor, breaker points and vacuum advance provides the spark timing. In a breakerless ignition, an electronic trigger and power transistor replace the breaker points. In a distribu