Very good. Good engineering logic and purpose. This is one for the achieves. When thinking about engines.
KRRon Sent: Friday, January 09, 2004 8:11 PM Subject: RE: KR>These 7 things: Auto vs Aero Engines for Aircraft > Serge and Colin and KRNetters, > I have really resisted hitting the reply button...But > I feel now is as good a time as any to reply on this > subject. > There are profound General Design differences between > automobile engines, and aeronautical engines. Which > make these engines very application specific. > > Let's start with basic components: > > 1.) Crankshaft-(Load Support) The Automobile engine's > crankshaft is is designed to turn a flywheel, clutch > and input shaft of a transmission(or torque > converter). Dynamic Thrust forces are relatively > small. More importantly, look how the automotive > engine handles or supports these loads. The flywheel > (clutch etc.)or torque converter is supported by the > crankshaft main thrust bearings and transmission input > bearings (front pump bearings for the automatic). > This allows the dynamicly loaded power application > device to be supported on both ends. In engineering > we call this simply supported. > > The Aero engine's crankshaft is designed to turn a > propeller. Dynamic thrust forces are enormous. The > aero engine doesn't have the luxury of a transmission > bolted to it to support the opposite end of the load. > This is why aero engines have very large thrust > bearing journals. This allows the dynamicly loaded > power application device(propeller) to be supported on > only one end. > In engineering we call this a cantalever. > To illustrate this point, place a board between two > saw horses. Place a weight in the middle of the board. > That's now a simply supported beam. Now remove one of > the saw horses. This is now a cantalevered beam. Keep > the board level. See what it takes to keep the ends > of the board level? This is how an aero engine > handles the load. The closer you get to the load the > easier it is to support it. > This is the same reason why aero engines have such > large thrust bearing surfaces. > > 2.)Cylinder heads. (Tolerances) Automobile engines > combine the combustion chambers into a single unit(s). > Aero engines use one cylinder head /combustion > chamber per cylinder. Automobile engine production > volumes will boggle the mind with the huge amount of > volumes each car company produces every year. Aero > engines volumes are a tiny fraction of what automotive > production volumes are. This isn't the only reason, > only part of it. Aero engines operate in a much > harsher environment than automobile engines operate > in. The aero engines tolerances are much closer than > automobile engines in order to get the expected life > from the engine. Tighter tolerances drive up cost. > The aero engine would not survive in it's harsh > environment if automotive production volume tolerances > were applied. The Individual Cylinder head allows the > aero engines deck height and therefore compression > ratio be tightly controlled. Even and smooth power > output is the end result. Automobile engines have > anything but even and smooth power output because the > compression ratio and deck heights cannot be closely > controlled, but rather compromised between the best > and worst deck heights, at best. Bores are typically > within .015 of each other. That's 10 times the > tolerance of an aero engines production bore > tolerance. > Do you know why Chevrolet finally stopped Corvair > production? It wasn't because of Nader, it was > because the engines were too costly to produce in the > needed production volumes. > > Ignition systems: (failure mode, redundancy & Time). > I hear this all the time folks complaining about > magnetoes, and how much better electronic ignitions > are. reliability etc. etc. Ever have a "Check Engine > Light" come on it your car when driving it? There's > plenty of cars on the shoulder because the engine just > quit. There are no shoulders to pull over on if the > electronic module quits on a flight engine. Ask > William Wynne, he does not advocate using an > electronic ignition on his Corvair Conversion. > Typically, when an electronicly controlled automotive > engine illuminates, the computer tries to retain the > last know set of variables, and goes into what's > called the "limp-in" mode. In an aircraft, if that > computer ever commanded a limp-in mode, guarrenteed, > you are not staying airborne. Failure mode of a > Magneto is a gradual performance degridation, which > allows the pilot to time to plan where he can make a > landing. Time. > Aero engines have to completely independant, > redundant ignition systems. Mags, wires and Plugs. If > you foul or burn a plug because the pilot wasn't > paying attention to his workload...You are more than > likely to suffer only a small degridation in > performance, again allowing: Time > An auto engine does not have independant, redundant > ignition systems. If you foul a plug, burn a rotor, > or chafe through a coil wire, you are in serious > trouble, and must take immediate action, because you > don't have: > Time > This is referred to in engineering as single point of > failure. There are too many single point of failures > in a single electronic ignition system. The same > thinking can be applied to electronic fuel injection: > Too many single point of failures. > Porsche experimented with a certify-able aero engine > I believe for Mooney?? It was a behemoth weight-wise. > and also a dismal failure. Why? because is had > redundant alternators, fuel injectors, ignitions, > computers and even a cooling fan... To get around the > single point of failure problem. > > An Aero engine operates in a completely different > environment than an auto engine operates in. The > differences in design, weight, systems, and even how > they are manufactured are profound. > Todays auto engines are even more application > specific, and are completely designed and optimsed for > a specific power-output, price range, fuel economy and > class of vehicles, even the kind of terrain they are > intended to operate in. > Aero Engines are designed for a specific output, > aircraft class, and are designed to turn a propeller. > Which means they too are designed to operate in a > specific kind of "terrain". > Because of these profound differences, converting an > automobile engine for aircraft use is possible, maybe > sometimes economicly feasable. But these significant > differences should be addressed, good conversions do, > however, a converted automobile engine will never > perform as well in an aircraft, as the aero-specific > designed engine will. Just as an aero engine doesn't > perform as well in a automobile as an automobile > engine will.
