S&S Engine Science- Part 2
Article And Photos By: S&S Cycles
Originally Published In The November 2016 Issue Of Cycle Source Magazine
In our last article, we talked to S&S Principal Engineer Roy Meyer, about structural and physical considerations in building big engines. In this article he delves more deeply into what it takes to really make that big engine perform like you would expect it to. A big inch engine has a lot of potential, and the actual performance depends on how efficiently we achieve that potential. That two major efficiency factors to consider are combustion efficiency and volumetric efficiency. Combustion efficiency refers to how much of the fuel is actually burned, and that depends largely on how well it’s vaporized. Liquid fuel does not burn. Fuel can only burn as a vapor. Vaporization of the fuel depends on the design of the intake, cylinder heads, and cylinder. In general, modern engines with appropriate camming and a properly sized intake system have pretty good combustion efficiency at about 90-95%. So vaporization of the fuel shouldn’t be a major concern. However, with too large an intake and the wrong cam, vaporization can be poor at all but high rpm. The air fuel ratio (AFR) refers to the ratio of the weight of air to the weight of fuel in the intake charge. Stoichiometry is the science of optimum ratios in chemical reactions. That’s where we get the term stoichiometric mixture. A perfect stoichiometric mixture for pure gasoline is 14.7:1. This ratio will theoretically allow all the available oxygen to react with all the available gasoline in a perfect engine. I Since our engines aren’t perfect and our gas isn’t pure, the actual AFR is slightly lower (richer). Performance engines may run an AFR as low as 12.5:1.
A slightly rich mixture doesn’t hurt performance very much, since you use up all the available oxygen and any unburned fuel is just expelled out the exhaust. A lean mixture hurts performance because there is excess oxygen available and you could be burning more fuel. Incidentally, while the stoichiometric mixture for gasoline is 14.7:1, stoich: for ethanol is 9.0:1 and nitromethane is 1.7:1. That explains why alcohol and nitro dragsters get such lousy mileage. Ignition timing also effects engine efficiency. Depending on the compression ratio, rpm, and the amount of turbulence in the combustion chamber, the amount of spark lead (advance) required varies a lot. High compression and an aggressive, turbulent chamber design will require less spark lead because the flame moves faster and does not require as much time to complete the burn. In fact, these engines will knock if too much advance is applied. At high rpm more spark advance is required to give the flame more time to burn the available fuel. If spark lead is not sufficient (retarded) the fuel may still be burning when it goes out the exhaust port. Not only does this cost performance, it also causes overheating. That energy is not being used to turn the crankshaft. It’s just heating up the exhaust port. Anyone for blue exhaust pipes? Volumetric efficiency is defined as the ratio of engine displacement to the volume of air the engine can actually take in.
It really only matters at wide open throttle (WOT) and at high rpm. If an S&S 124-inch engine is idling, the throttle is almost closed, and it’s taking in very little air. Therefore, the volumetric efficiency is very low . . . as is power output. At high rpm and WOT, the engine is cranking out some ponies, and volumetric efficiency is much higher. How high? If the manifold pressure is the same as atmospheric pressure when this engine is screaming at WOT, volumetric efficiency is very close to 100%. If manifold pressure is less than atmospheric, volumetric efficiency is less than 100%, indicating a restrictive intake system or one too small for the engine. That’s why S&S makes larger carburetors and throttle bodies, and why we offer cylinder heads with high flowing ports. With optimal valve timing it is possible to achieve volumetric efficiency over 100%. That means you’re stuffing more air into the engine than its displacement volume. That’s exactly what a turbocharger or supercharger does, but 100% + is achievable in naturally aspirated engines through cam timing, intake design, and exhaust design. To understand how this works you need to realize that an engine is a dynamic system.
Air doesn’t smoothly flow into the air cleaner and out the mufflers. The flow of air is intermittent, and energetic events are involved. As a result, harmonics, resonances, and pressure pulses are generated that effect how the engine runs, at specific rpm. Physical dimensions and material characteristics of the engine components cause them to resonate at specific frequencies. Finally, air has mass. That means it has inertia and momentum. It takes energy to make it move, and once it’s moving, it keeps moving. How can we put all that to work? First of all, on the exhaust stroke, you have a pipe full of air rushing away from the cylinder head. Even when the piston reaches TDC the air keeps moving crating a slight vacuum in the chamber. If the intake valve is open at this point, air from the manifold will flow into the chamber before the intake stroke even begins. On the intake stroke, air rushing into the cylinder through the intake tract, will continue to flow even after the piston reaches BDC and starts back up for the compression stroke. All it takes is to keep the intake valve open for a little while longer and we get some extra air in the cylinder. Both of these occur at a specific rpm, and generally at high rpm, but that’s how it is possible to achieve 100%+ volumetric efficiency. In the next article Engine Science Part 3 we’ll explore how it works in the real world. We don’t have total control, but we’ll talk about how to make a silk purse out of a sow’s ear.