Corvette Engine Basics
With the exception of the ZR-1's LT-5 engine, all C4 Corvettes use a 350 cubic inch displacement motor with 8 cylinders and 16 valves.
98% of all gasoline powered automobiles and 100% of all Corvettes use a combustion process called the 4 stroke system to extract energy from the burning of an air/fuel mixture and that system is composed of 4 separate, but equally important parts: the Intake, Compression, Power and Exhaust strokes (more colorfully referred to as Suck, Squeeze, Bang and Blow).
This fact sheet looks at the 4 stroke system and the engine processes that are in play when your Corvette is flying down the road.
Internal Combustion Engines
Yes, there is an External Combustion Engine: Think Stanley Steamer and you will understand. A fire, external to the motion producing process, heats up a boiler, the resulting steam turns a flywheel (which is coupled to the drive wheels) and off you go.
Internal combustion on the other hand places the fire inside the engine and the result is an expanding gas that forces a piston to move. The piston is coupled to a device that converts the linear motion (the piston's connecting rod) to a rotating motion (the engine's crankshaft) which in turn is coupled through clutches and gears to the drive wheels and, again, off you go.
Managing this process is the tricky part and the subject of this discussion.
Releasing Energy to Accomplish Work
Pour a teaspoon of gasoline on the ground, step off a safe distance and throw a lighted match into the puddle and you will start a process whereby the liquid is converted into a gas releasing heat in the process. Put a boiler above the fire and you have the beginnings of the external combustion engine described above. All that remains is to couple the resulting steam to a flywheel. In this case, the heat is the product and the expansion of the burning gas is the byproduct.
Fine, thanks for the history lesson but what about internal combustion engines?
OK, fair enough.
Atomize the gasoline into extremely small droplets, mix in the proper amount of oxygen and confine the mixture to a small space that has every portion composed of strong, non-yielding metal except one side which is free to move a carefully designed distance.
Ignite that mixture and the burning gas will expand. As it expands it will force the unrestrained part (the piston) to move to the limit of its allowed travel. The expanding gas is now the product and heat is the byproduct, the opposite of the external combustion engine.
The Basic Engine
An automobile engine needs the following to produce power:
A device to mix fuel and air together into a combustible mixture. Item 1 is either a carburetor or fuel injection system.
A source of ignition.
Some means of timing the exact point when the ignition of the mixture should
occur.
A chamber to contain the burning mixture.
Something to alternately seal and unseal the chamber where the fuel is to
be burned.
Devices to convert linear motion to rotating motion (Except for rotary
engines like the Wankel).
Item 2 is normally an ignition coil.
Item 3 is a distributor or similar device.
Item 4 is the engine block with its cylinder bores and heads.
Item 5 are the intake and exhaust valves and the camshaft
Item 6 are the pistons, connecting rods and crankshaft.
Different Strokes
In the 4 stroke engine, the crankshaft rotates twice for each occurrence of power being produced by any given piston. Each rotation is an equally important part of the process.
Intake Stroke:
The first stroke is the intake stroke and during this time, the air/fuel mixture is drawn into the cylinder. The crankshaft rotates 180 degrees during this portion of the cycle.
As the piston is pulled downward by the rotating crankshaft, a vacuum is created above the piston and when the intake valve is opened by the rotating camshaft, the air/fuel mixture rushes in from the intake manifold where it was been parked by either the carburetor or the fuel injection system.
This vacuum is the source of the intake vacuum that must be present for the engine to run at all. (To start successfully, about 1 inch of vacuum must be present but once started, the vacuum varies from a few to several inches depending of engine load and RPM. A vacuum gauge can be attached to the intake plenum on a C4 Corvette and should show steady vacuum when the engine is operated at a constant RPM/load condition).
When the piston reaches near the bottom of its allowed travel, the intake valve closes and the next stroke of the cycle begins.
Compression Stroke:
As the crankshaft continues to rotate (another 180 degrees), the piston begins to move towards the heads and as a result, the air/fuel mixture is compressed into a smaller and smaller space. The amount of change is the compression ratio and a 10:1 ratio (for instance) means that the normal atmospheric pressure relationship of the air and fuel molecules is changed so that they are compressed into a volume that is 10 times less than is normally the case in free space. The result is an amplification of the available energy.
Power Stroke:
Near the point where the piston is at the top of its movement, the ignition system sends a jolt of several thousand volts to the spark plug. The voltage jumps a gap on the plug, ignites the fuel/air mixture and the expanding gas forces the piston downward which rotates the crankshaft another 180 degrees. Of the four strokes, this is the only one that produces any power.
Exhaust Stroke:
When the piston reaches the bottom of its allowed travel, the exhaust stroke begins. The exhaust valve opens and as the crankshaft rotates still another 180 degrees, the hot gases are forced out of the cylinder into the exhaust manifold and from there to the catalytic converter, mufflers and eventually to the atmosphere.
In a Corvette engine, there is obviously more than 1 cylinder present and each of the pistons go through the same 4 strokes but each piston's motion is offset from the others (with respect to the crankshaft's rotation) so that power is applied to the transmission in a smooth manner giving the illusion of a continuous production of power instead of a series of separate events.
Timing:
According to the old saying, timing is everything and, in an automobile engine, that is certainly the case. In the basic engine, there are two areas where timing is involved: The opening and closing of valves and the point at which the spark plugs are fired.
Valve Timing:
The camshaft has lobes or built up areas that force a push rod to move up and down. The push rod in turn forces the valve to open and a spring forces the valve to close again once the cam has rotated past the point where it opened the valve.
Many factors are involved in when the valve opens, how long it stays open, how fast it opens and closes and finally how far it opens. For the purposes of this discussion, just realize that the lobes on the camshaft are what causes the valves on each cylinder (both intake and exhaust) to open and close and the timing of the opening and closing is tightly controlled to (A) prevent damage to the piston as it moves and to (B) maximize performance of the engine.
Ignition Timing:
On the surface, you would think that the proper time to fire the plugs is just as the piston reaches the maximum amount of upward travel but actually, a more efficient burn occurs when the spark plug is fired before the piston reaches the top of its movement (Top Dead Center).
The reason is the mixture does not explode but rather is burned---at a rapid rate to be sure, but still burned---and this takes time.
When the spark plug fires, a flame front advances across the top of the piston from the ignition point. The most efficiency comes when the flame front advances smoothly and uniformly across the piston and there is time to convert the maximum amount of the mixture charge into an expanding gas. Heat is a by-product so generating the maximum amount of heat is not the objective.
Maximum power comes when the temperature of the burning charge is just a bit less than the absolute peak cylinder head temperature (referred to as Rich of Peak) but the peak temperature is where the maximum amount of the mixture is burned, a good thing from an emission standpoint and also what produces the best mileage.
In the simple engine model we are discussing here, the air/fuel mixture would be adjusted for the smoothest running engine if there were no instrumentation present to do it precisely and that situation would occur somewhere between the max power point and the max efficiency point.
In a computer controlled engine, the timing is adjusted to yield a 14.7:1 air/fuel mixture which is the optimum for holding down pollution. It also is close enough to the max power point so as to be not worth fooling around with except in pure racing situations.
The exact time to fire the plug is controlled by the distributor or similar device (Opti-Spark system in the LT-1 Corvette engine for instance) and the degree of spark advance is not a static point in time but rather is dynamic point depending on engine load and RPM. In the C4 Corvette, the ECM/PCM (Engine Control Module/Powertrain Control Module) monitors several sensors to determine exactly when to fire the spark plugs.
BC engines (Before Computers) used a system called vacuum advance which sampled intake vacuum since there is a direct relationship between the amount of intake vacuum and the load being placed on the engine. It was a crude but effective way to control how far before Top Dead Center the plug fired on each cylinder.
Summary:
The above is a basic primer of how a 4 stroke engine works. When you throw a computer into the mix, nothing really changes except the computer looks at various sensors and, using actuators, changes the spark timing and the air/fuel mixture hundreds of times per minute as it tries to always keep the air pollution at the minimum level.
Instead of a distributor with vacuum advance, the computer controls the amount of spark advance. Instead of a mechanical system to control fuel delivery, the computer tightly controls the on and off time of the fuel injectors. Instead of just approximating the amount of air present in the intake manifold, the computer precisely measures it and adjusts the air/fuel mixture accordingly.
Modern engine systems like the Exhaust Gas Recirculator (EGR), Air Injection Reactor (AIR), the PCV valve, Catalytic Converter etc., are all simply devices used to minimize air pollution. They contribute nothing to engine performance except to lower it from what the engine could produce if engineered without regard to pollution. You can't remove them however because the modern automobile engine has become a closed loop system, tuned for maximum performance and efficiency with all the smog devices in place and functioning properly. As a matter of fact, when one or more of those sensors malfunctions or indicates that something isn't right, the computer notes that fact and reacts accordingly. If things are far enough outside the design parameters, a trouble code is stored and the "Check Engine" or "SYS" light will quickly get your attention.
On a Corvette, as long as you operate at less than 80% of wide open throttle (WOT), the computer tries to keep the pollution at a minimum level. Above 80% WOT, all bets are off emission-wise and maximum performance is the goal. The mixture is adjusted to the max power ratio and you are launched!
Defeating the emission control systems therefore makes no sense considering you have generally floored the accelerator anyway when you race.