Comments by "Keit Hammleter" (@keithammleter3824) on "VisioRacer"
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VisioRacer is quite right - inline engines have more of a problem with crankshaft torsional vibration. It wasn't a problem with the old American inline 8 car engines due to low compression design, and low power output.
In the 1930's America, a long engine bay on a car was a status symbol. Post-war, long engine bays were considered ugly, and women drivers found long cars hard to park. So engines had to be shorter - hence V8's, not I8's, post war.
An additional minor advantage of V-engines is lower friction. For any given cylinder swept volume, you would expect the power lost in friction would be directly proportional to the number of cylinders. However, for any given cylinder size and number of cylinders, the V-formation has less friction than the inline form due to the staggering of peak loads on each crank throw.
For a while. I worked as the engineer for a dealer selling large industrial diesel engines. Over a whole range of an engine series, the cylinder size is always the same. One series we sold gave about 50 kW per cylinder, so if you needed 200 kW, you got an inline 4, if you wanted 300 kW, you got an inline 6, if you needed 400 kW, you got a V8, and if you needed 600 kW, you got a V12. And if you needed 800 kW, you got a V16. The V8 got the same size starter motor as the I4. The V12 got the same size starter motor as the I6. Of course the V16 had two starter motors fitted, each the same size as the one fitted to the V8 and I4. This is not the full picture though - for example the I4 cranked a bit faster that the V8. But the friction loads were close enough to allow starter motor standardization.
The V engines needed only slightly larger starting batteries too. So, all up, a V8 is cheaper than an I8 for example.
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@bruceparr1678 You are misled. Two important factors about efficiency:-
# Aircraft engines could indeed be quite a bit better than 40% thermodynamic efficiency because they are designed to run at a specific RPM (usually about 2000) - thus turbulence, which is set by cylinder head and valve geometry, can be optimised. A car engine, and to large extent a truck engine, needs to operate over a wide range of RPM, so turbulence is necessarily a compromise. If it is enough at low RPM to prevent pinging, it is too much at high RPM.
# Since the amount of heat lost is proportional to cylinder bore, but power is proportional to the cube of bore, it follows that, within reason (considering factors like con-rod mass), the bigger the bore the better. Those turbo-compound aircraft engines had cylinder capacity around 2.5 to 3 litres. Given that for car engines 3 litres is a moderately large size, if a car engine was optimised for highest thermodynamic efficiency, it should have only one cylinder. This would confer totally unacceptable vibration. And it would need to be large and heavy. 4 cylinders is about the minimum number of cylinders for a smooth ride in a car, 6 cylinders is better and that means the bore size has to be well under the optimum for efficiency.
V8's were designed for cars, to further reduce vibration, and get more capacity in a short length, but there was, compared to 4 or 6 cylinders with the same total displacement, a fuel consumption penalty.
When BMC were doing the initial design of the Mini Minor, they calculated that about 800 to 900 cc would give the performance needed. So they designed and prototyped a 3-cylinder engine in order to not take the bore too far away from the requirements of efficiency. On test they decided that vibration would be too high for market acceptance and stayed with 4 cylinders.
You should note that supercharged, turbocharged, and turbo-compound aircraft engines were set up so that at sea level the amount of boost was minimal or non-existent. The system was set up so that a near constant amount of air-fuel mix was pushed into the engine regardless of altitude, i.e., by means of waste gates and other means, the boost increased as altitude lowered atmospheric pressure, so the engine was operating at sea level conditions even at maximum altitude.
Note also that with aircraft engines, a couple of factors gave a few more percent efficiency that don't apply to car and truck engines. At altitude, air temperature is much lower. Engines thermodynamically work on the difference between combustion temperature and air temperature, so you gain a bit of efficiency at altitude. Secondly, the exhaust gasses act like a jet engine and impart a bit of thrust to the airframe. It's small but it does count.
A typical car engine has a thermodynamic efficiency of around 22 to 26%. If there was a way to easily increase it to 40%, manufacturers would have long since done it, and we would all be getting 40 - 50 miles per gallon. Fact is, there isn't.
Except for one thing: If the vehicle is a hybrid, i.e., an engine driving a generator charging a battery, the battery in turn feeding an electric motor driving the wheels, the engine can be optimised for a specific RPM and always operated at that RPM. But would you want a car where the engine is always screaming at high RPM, regardless of how fast or slo you are driving?
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