Draft Version 0.9
|Intake||Down||Intake air/fuel is drawn into the cylinder||Open||Closed|
|Compression||Up||The intake charge is compressed||Closed||Closed|
|Power||Down||The charge is ignited||Closed||Closed|
|Exhaust||Up||The spent mixture (exhaust) is discharged.||Closed||Open|
This cycle requires 2 crankshaft revolutions to complete. The camshaft, which is operating the valves, will only complete one revolution during this cycle. Therefore, the cam drive is a 2:1 ratio with the crankshaft (2 crank revolutions per cam revolution).
The key to enhancing engine power is to efficiently fill the cylinder with the maximum amount of intake charge as possible and efficiently combust it. Spark ignited (SI) engines require a certain air to fuel mixture ratio to operate properly. More fuel requires more air to keep the proper mixture and burn completely. An ideal theoretical engine would operate with a stoichiometric (just enough air to allow complete fuel combustion) air to fuel ratio. The intake charge is this air fuel mixture. There are several things that affect how much charge you can get into the cylinder.
Optimizing camshaft design for mid to upper RPM ranges will result in poor idle quality. The amount of overlap needed for enhanced performance at the higher speeds will allow exhaust to flow back into the cylinder during overlap due to intake manifold vacuum, and lack of any significant scavenging or inertia affect, at idle. This exhaust gas is inert in the combustion chamber. The poor idle cannot be eliminated by air/fuel mixture changes.
For multiple cylinder engines, and low to mid RPM range performance, you can further optimize the intake system by combining runners. This will let some of the inertia from one cylinder be used by another. Runners can be combined into one final plenum which can also enhance intake inertia. Back in the carburetor days, the dual plane intake manifold used this concept.
Many modern engines use 2 intake runners per cylinder. This began with multiple intake valves (4 valve engines). Each intake valve would have itís own intake runner. One intake runner would be long to enhance low RPM performance. The other intake runner would be much shorter. The short runner is closed by a small throttle plate on that individual runner until higher RPM, usually around 3,000 to 4,500 RPM. This method gives you the best of both worlds. These engines have the capability of having a much flatter power curve. The valves are usually the biggest restriction in the intake system. Having 2 somewhat smaller valves allows more flow. Because the pistons are usually round, the combustion chamber has limited space to put the valves. If you use more than one intake valve, you can achieve more total opening area and reduce the restriction substantially. Because of this, 3, 4, and 5 valve per cylinder engines build more power higher in the RPM range.
The multiple intake runner concept can also be employed on normal 2-valve engines. Ford has done this on many engines and they refer to it as split-port induction. The 1997 F150 V6, the 1997 Escort 2.0L, and the 1997 Windstar 3.8L used this technology.
Sonic affects can also be used to enhance performance. Then the intake valve slams shut, a sound wave echoes back up the intake runner. Sound a basically a high frequency pressure pulse. If you time it right, and use resonance to your advantage, this sonic wave can be used to pressurize another cylinder. Chrysler used this technology back on the 1960ís.
To better understand the inertia effects, you can use water flows. Consider water hammer. Then you flow water through a pipe at high speed, and then slam the valve shut rapidly, the inertia of the flowing water will hammer the valve causing a bang sound. If you put a pressure gage on the tube just before the valve while you do this, you will see a huge pressure spike. Inertia affects any mass whether it is fluid or solid. When you throw a ball it requires force to accelerate the ball from rest. Once the ball is in motion, it requires force to stop it (catching the ball).
The tubes for each cylinder to the collector are called primary tubes. These primary tubes can be sized, both diameter and length, to optimize the exhaust inertia while keeping minimum restriction. It is a trade-off and will only be optimum at a specific RPM and load. This is very similar to optimizing the intake system and the same key things apply. For low RPM performance, a small diameter, long tube will give the best port velocity, and exhaust inertia. For midrange performance, a shorter length, larger diameter runner is best. For very high RPM, a very short, large runner is needed because restriction is the biggest problem at high RPM.
By grouping primary exhaust tubes into a collector, the vacuum pulse from one cylinder can be used to create a vacuum on the other exhaust ports. This vacuum will cause the exhaust to begin flowing more quickly as the exhaust valve opens. The collector must be long enough and small enough to keep the inertia of the exhaust pulse optimized while keeping back-pressure low.
Back-pressure is something we are stuck with, not something we want. By optimizing the inertia affect, some back-pressure will be realized during the peak of the exhaust pressure pulse. It is not something you want but rather simple physics.
Once you get past the end of the collector, you want to release the exhaust with as little restriction as possible. The end of the exhaust header is not really the end of the collector. The collector area in the exhaust should usually be about 18" to 24". If you have 2 banks of cylinders with each bank having itís own collector, you will also have alternating pulses from each bank. In order to reduce restriction, you should place a tube between the banks at the end of the collector area. This will allow pressure pulses to swap back and forth which will even them out and allow smoother flow through the rest of the exhaust system.
At very high RPM, the restriction becomes a much bigger problem. Because of this, attempts to utilize exhaust inertia are generally abandoned in favor of reducing restriction.
Combustion chamber design makes a big difference in the octane requirement. In theory, 87 octane fuel can only support compression ratios up to about 7:1. In reality, it can be much higher. There are a few factors that affect this. It is very important that the air fuel mixture burn quickly and orderly. If the flame path is too far, the mixture can actually hit the point of detonation prior to the flame front causing detonation. A good combustion chamber design will have a short flame path and have very high mixture turbulence at the time of ignition. This is where the terms "high-swirl" come into play in modern engines. The turbulence can come from intake swirl, combustion chamber "squish", or both. Some modern engines can support very high compression ratios, or effective comression ratios, with much lower octane fuel. In some cases, like the Porsche 911 turbo, 93 octane (R+M/2) fuel can support up to 17:1 effective compression (9.4:1 with 12psi boost). Effective compression for boosted engine can be calculated by the following equation: [(absolute intake pressure)/(absolute barametric pressure)]x(compression ratio). Jaguar experimented with compression ratios as high as 14.5:1 on 93 octane fuel but the NOx was too high to pass emissions. Higher engine speed will also reduce the octane requirement. Lower air and engine coolant temperatures will reduce the octane requirement as well. For more information about octane requirements, see http://www.faqs.org/faqs/autos/gasoline-faq/part3/section-1.html.
A high overlap camshaft will generally let you run a higher compression ratio without pre-ignition or detonation due to the EGR affect and poor filling at low speeds when the detonation tendency is the greatest. Detonation can also be reduced or eliminated by retarding the ignition timing.
Compression is affected by the piston design, head gasket thickness, and the combustion chamber volume. The crankshaft and the connecting rod length can also affect the compression ratio by moving the piston travel up in the cylinder.
Turbochargers work very well on diesel engines because there is no throttle (on most diesels), the exhaust flow rate is high even at idle. This keeps the turbo shaft speed high so it can quickly build boost. At highway speeds, the turbo can build boost even when going a steady speed. This increases the air entering the engine. This additional pressurized air allows for more complete combustion. Diesels do not use a fixed air fuel mixture. They will have extra air until full throttle and sometimes even at full throttle. At full throttle, it can be near or slightly richer than stoichiometry (this is called over-fueling). The turbo allows the diesel engine to add more fuel with the additional air for more power. At steady speeds, increased fuel economy will be realized due to the additional air, and air pressure, allowing for more complete combustion and higher affective compression.
On spark ignited engines, the turbocharger does not give the fuel economy benefit it did on the diesel. SI engines still need the stoichiometric air/fuel ratio. They also have a throttle to reduce the intake airflow (and pressure) to control engine speed. Without a throttle, SI engines would run at full power all the time. Since the throttle limits the intake air, the exhaust flow is very low at idle. This allows the turbo to spin very slow (relatively speaking). If you try to apply the throttle rapidly, the turbo will need to spin up. This takes time and is referred to as turbo lag.
Nitrous oxide is very effective. It will produce power gains across the RPM range and has no back work. Actually, the gains are bigger at lower RPMís. Nitrous oxide kits generally have a set amount of nitrous oxide and fuel they add to the engine. Because of this, they produce huge gains at low RPM because the same amount of nitrous/fuel is added no matter what the RPM. Some nitrous kits will have multiple stages to compensate for this. Nitrous kits are rated by the added power they produce. This added power is generally realized at nearly all RPM levels in the power-band of the engine.
Nitrous oxide is under high pressure. When it expands, and changes states from liquid to gas, it cools significantly. This is the opposite affect as compressing air. Compressing heats, expanding cools. This cooling affect is also beneficial. By cooling the intake charge, it becomes denser. That means that a given volume of intake charge contains more oxygen, again allowing for more fuel, and therefore more power.
Hypereutectic pistons may have problems with larger nitrous oxide systems. It is best to use a good forged piston and O-ringed head when using large nitrous kits.
Performance engine design must consider all aspects of the camshaft, intake system, and exhaust system. When I say intake and exhaust systems here I am including the portion of those systems in the cylinder heads. The engine will only perform as good as the weakest link. If you have a high RPM camshaft and exhaust design, and a low RPM intake system, your performance will be sacrificed at both ends of the power curve. When designing a performance engine, one must consider where in the rpm range they want the power to be optimized and design the entire system around that. If you want a real wide power-band, supercharging or turbocharging is the best option. If you want gobs of power in short spurts (drag racing or mud bogging for instance) nitrous oxide is an excellent way to get the job done.
to main page