| Stroke | Piston Travel | Description | Intake Valve | Exhaust Valve |
|---|---|---|---|---|
| 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 |
While there a many factors affecting engine performance, only the most significant ones are covered here. Other factors include friction, combustion efficiency, ignition, moments of inertia, and lubrication.
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.
The camshaft is a key part of getting maximum charge into the cylinder. By increasing valve lift, the intake charge has less restriction and will fill the cylinder more completely before the intake valve closes. This is a fairly simple and intuitive concept. The affect of overlap is a bit more complicated. Overlap is the time when both the intake and exhaust valves are open somewhat. The exhaust valve is partially closed and the intake valve begins to open. If you have good exhaust scavenging (covered later) this last portion of the exhaust stroke will actually be sucking the exhaust out of the cylinder. This sucking will help draw the intake charge into the cylinder sooner if the intake valve is open. Performance cams will also leave the intake valve open after the piston begins the compression stroke. If the intake manifold is properly designed to work with the cam, the intake charge in the intake port will gain inertia when it is drawn into the cylinder. This means that it is now moving and force will be required to stop it. When the piston begins the upward stroke, the intake charge in the manifold runner will need to stop. Because of the inertia, it pressurizes instead. If you time the valve operation just right, you can use this pressurizing affect to its full extent before finally closing the intake valve. This affect is usually maximized at mid and upper RPM ranges and the cam and intake must be matched to optimize the same rpm range.
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.
As we discussed in the camshaft section above, the intake system needs to be optimized to match the cam profile. For low RPM engines, long, small diameter runners will cause the maximum port velocity, and thus the most intake inertia. These long small runners will become more and more restrictive as the RPM increases since the airflow will increase with RPM. Therefore, a midrange RPM manifold will have shorter but larger diameter runners. As the RPM goes up, the port diameter can increase to keep the optimum port velocity. Once you get really high RPM, restriction becomes the biggest problem. At this point, you generally need a short, large diameter port.
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 it's 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 1960's.
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).
It may seem intuitive that bigger, more open exhaust would be the best for engine performance. Some believe that there is a proper amount of exhaust back-pressure for optimum performance. Both of these theories are wrong. There are a few key things that make exhaust flow different than other fluid flows. First of all, the exhaust is coming out in pulses. When each cylinder exhausts, a large surge of flow enters the exhaust system. Then it stops again until the next exhaust stroke. True, larger exhaust ports and tubes will flow better but flow is not the only thing to consider here. If you size the exhaust tubing properly, you can take advantage of the inertia of the exhaust pulses similar to what can be done on the intake side. When the cylinder exhausts, the pulse builds a pressure in the exhaust tube. Once the valve closes, this exhaust pulse is traveling down the tube with velocity. Because of this, a slight vacuum is created at the valve. This pressure-then-vacuum pulse will travel all the way through the exhaust system assuming the restriction is low and the tube diameter does not change much.
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 it's 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.
Compression ratio is also an important factor in engine performance and efficiency. The higher the compression ratio, the more thermally efficient the engine. Thermal efficiency is the ratio of energy in divided by energy out. Fuel has a certain amount of energy that is released during complete combustion referred to as heating value). This heating value varies depending on the products of combustion and the state of those products. Because engine exhaust is significantly above the boiling point of water (one of the primary products of hydrocarbon combustion), only the lower heating value is realized. The limiting factor for compression ratio is the fuel. If your compression is too high, the heat caused by compressing the intake charge will actually ignite it (pre-ignition). When air is compressed it heats up. This heat of compression concept is what makes diesel engines run. Sometimes the intake charge will not pre-ignite but, due to high temperatures, it will detonate when ignited by the spark plug. Common gasoline can only handle a certain amount of compression or it will pre-ignite, usually about 10.5:1. Higher octane rated fuel can handle more compression, up to around 13:1. Alcohol can handle even more, up to about 15:1. Diesels use compression ratios from about 16.5:1 to 23:1.
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.
The cylinder heads are part of both the intake system and the exhaust system. The heads are usually the weakest link in these systems and therefore your best area to improve them. The runner size should be optimized just like the rest of the intake and exhaust systems covered above. In addition to runner size, the area just behind the valve (the bowl) is critical. Optimum intake port velocity really aids in cylinder filling. The valve size is also key. Reducing restriction in the valve opening area with a 5-angle valve grind can yield some surprising results.
Since engine power is directly related to the amount of intake charge you can pack into the cylinder during the intake stroke, force feeding the engine can give you big gains. However, this comes at a price. Work is required to pressurize the intake charge. This is referred to as back work. Superchargers are belt driven and do require significant power when they are pressurizing the intake charge. However, the power gains are substantially greater than the back work. Therefore, you do realize a significant net gain. If your engine is set-up properly, and your boost pressure high enough, your cylinder will actually be pressurized during the intake stroke. This means that the cylinder is actually applying a little power to the crank instead of drawing power from it. Because of this, a misfiring supercharged engine is much smoother than a misfiring naturally aspirate engine under full load.
Turbocharging has the same affect on the intake charge as supercharging. However, turbochargers use exhaust pressure to drive the compressor. The back work comes from the exhaust pressure. The piston will draw power from the crank to force the exhaust out into the pressurized exhaust manifold. The exhaust side of the turbocharger uses this pressure to spin the exhaust wheel on the turbo. This exhaust wheel is connected with a shaft to the intake wheel. The intake wheel then compresses the intake charge.
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 not a fuel, it is an oxidizer. Air contains less than 20% oxygen, which is the only component of air used in combustion. The amount of fuel must be balanced with that oxygen for proper combustion. However, if we have a higher concentration of oxygen available for combustion, we can also add more fuel. The same volume of intake charge can have much more power. Say for instance we add enough nitrous oxide to bring the concentration of oxygen in the intake charge up to 40%. Now we can add twice the fuel and the combustion will produce twice the power to the crankshaft. The key to nitrous oxide is to make sure you add enough extra fuel. If you have a high concentration of oxygen available, and you run the fuel mixture too lean, you can cause major engine damage. You must add extra fuel when adding nitrous oxide. All nitrous kits have some method of adding the extra fuel. Most have a fuel solenoid and nozzle to inject the extra fuel.
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 RPM's. 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.