Large Radial Engines Were About As Good As Can Be?

GregP said:

Modern gasoline radials aren't even close. The Rotec 9-cylinder radial is 220 cubic inches and makes 150 HP for 0.68 HP/cubic inch. The Vedeneyev M-14-P has 9 cylinders, 621 cubic inches, and makes 400 HP for .644 HP/cubic inch. These are very reliable and desirable engines for aerobatic aircraft. They now come with either air-start or electric start as an option. One reason they aren't as close to the magic 1 hp/cubic inch is we are limited to about 100-Octane LL fuel today. There isn't any 145 / 150 PN leaded fuel around except in special batches sometimes made for the Reno air races and whatever brews are made up in private hangars.

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The Lycoming IO-360 makes 200hp in certified form, which gives 0.55 hp/ci (up to 220hp for experimental: 0.65 hp/ci). Gives some idea of what modern aircraft engines are giving out.

Lycoming got up to 0.71hp/ci back in the 50s on a supercharged (mechanical) geared 480 cu in engine (340hp from take-off to 8,000ft) using 100/130 fuel and 9lb of boost.

There were a few other models/sizes of supercharged/geared engines.

They went away when they found that engines with more displacement per cylinder could make the same or similar amounts of power while running slower and not needing the reduction gear and/or supercharger. The larger engines lasted longer between overhauls and without superchargers and reduction gears they were cheaper to overhaul.

The gas turbines killed the supercharged high rpm flat six/flat eight market.

A lot of things affect engine choice in the light airplane market. HP per cubic inch is way down on the list. somewhere about paint color on the engine block

="The kommandogerat is a tinkertoy compared to the mechanical control systems you can rustle up now."

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In what way?

Other than being instantly programmable (i.e. being altered in 5seconds instead of needing a new cam ground) it does exactly the same thing.

It allows ignition timing, boost and fuel to be altered on the basis of inputs of temperture, pressure, and load. Most WW2 engines were NOT limited in performance
by virtue of over simplified control systems. They were limited by all the fundamental physics that wasnt yet sorted out, mostly combustion system
understanding.

All the tiny tweaking of transient performance is meaningless in a virtually constant speed engine like aero-engines.

The engines in the lab in 1940 were instrumented to similar degrees as they are now, cylinder pressure, EGT, exhaust gas analysis, albeit very laboriously and without nice logging, but the kommandogerats (etc) were not fred flintstone items and although of course you`d chuck them in the bin now, throwing them away will only gain you convenience not performance.

Shortround6 said:

Lycoming got up to 0.71hp/ci back in the 50s on a supercharged (mechanical) geared 480 cu in engine (340hp from take-off to 8,000ft) using 100/130 fuel and 9lb of boost.

There were a few other models/sizes of supercharged/geared engines.

They went away when they found that engines with more displacement per cylinder could make the same or similar amounts of power while running slower and not needing the reduction gear and/or supercharger. The larger engines lasted longer between overhauls and without superchargers and reduction gears they were cheaper to overhaul.

The gas turbines killed the supercharged high rpm flat six/flat eight market.

A lot of things affect engine choice in the light airplane market. HP per cubic inch is way down on the list. somewhere about paint color on the engine block

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Reliability is a lot of what killed the GO-480. I haven't heard much good ever said about them.
A much longer-lived geared engine the continental GO-300 makes 175 hp, which equates to 0.58.

For an air-cooled piston engine, displacement directly relates to surface area and cooling efficiency, so its not entirely irrelevant. It looks like somewhere around 0.6 is around the sweet spot for reliability, for the tech of the time.

Gas turbines killed off the high Hp piston engine market. They still can't really compete under about 300Hp.

Shortround6 said:

What is truly amazing is not that we can do better now, but that they were building hundreds or thousands of engines per month that were equivalent in HP to weight and fit and finish to the race car engine of the 1960s which were never built in anywhere near those quantities ( 25 engines a year might be high production for a race car engine)

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People underestimate how good piston aircraft engines were and even are compared to race car engines. While I don't think radials were as good as they could be, they were as good as they could be within the technological and economic constraints in force at the time. Now, of course, an ab initio piston aircraft engine of over a few dozen horsepower would probably be a diesel (avgas is nearly unavailable in many parts of the world).

Using modern design technology but 1945 materials for a radial would likely permit a far better supercharging system (two stage turbocharging, perhaps?) and greater compression ratios with the fuels of the era (or lower performance number fuels for the same compression ratio), lower sfcs, and less structural weight.

I sure don't know enough but as always, there are a bunch of trade-offs. Wright and P & W both used hemi heads (or darn close to it) with pushrods and spark plugs near the center of the head, one plug on each side of the valves. there are certainly a lot more sophisticated head designs and valve arrangements BUT.......................

Does the more sophisticated valve arrangement weigh more?
Does the more sophisticated valve arrangement require more volume/increase diameter?
Does the more sophisticated valve arrangement cost more?
Does the more sophisticated valve arrangement take up valuable real estate that could be used for cooling fins?
Does the more sophisticated valve arrangement actually allow the engine to turn more rpm or is the rpm limit restriction due to the bottom end?

There may be more. Please note that some of these considerations conflict with each other, A radial engine using high boost needs a high octane fuel, It also needs a lot of cooling fin area. If you use a valve arrangement that cuts down on the cooling fins you may not be able to use as much boost which requires higher rpm for the same power (except you still have to get rid of the heat.) and similar tail chases.

The amount of cylinder finning is going to directly affect how much power you can make in each cylinder regardless of how it is made. Fans may help (but if the plane is cruising at 300mph needing a fan seems ridiculous) but they require power and maintenance.

Modern technology can do a lot but lets look at the goals they were meeting and how it was done.

WW2 piston engines were clearly capable of further improvement and some lateral thinking in the day.

Here are several of the technologies that are common today that could have been introduced in the early 40s.
1 Variable Length Intake Ports. The From the DB601E Daimler Benz Introduced a radical valve overlap to resonance scavenge the cylinder head of end gasses and pressurise it by placing an node there. The variable length was necessary to ensure the engine idled and started smoothly. Direct in cylinder injection prevented fuel loss.

It should be noted that the excuse given for Rolls Royce not introducing this system that it would give up the charge cooling effect of injecting the fuel into the supercharger (precooling it) was overcome in the BMW801D2 from late 1943 onwards when 'rich mixture' injection was introduced to provide emergency power by injecting the fuel into the eye of the supercharged during WEP.

2 Variable Valve Timing. The BMW802 18 cylinder radial was to be available with variable valve timing on the exhaust. This achieves the same effect as changing the inlet manifold length. Engines like the R3350 needed fuel injection anyway due to the fuel distribution and fractionating issues they faced.

So the above are clearly possible but never applied to allied engines.

3 Knock sensors. This can be implemented in and around WW2. The older Waukesha CFR test engine simply used a pin that bounced up and down when knock was detected. It was visually extremely obvious. This itself could be used to drive a small electrical motor to advance or retard the ignition. A knock sensor is really just a microphone/accelerometer.

Knock sensors could be introduced to retard ignition (with an electrical motor) when knock is detected and if a 10% retardation of ignition doesn't stop the knock the boost can be reduced.

This allow the engine to operate with much higher boost settings or delivered with higher compression ratios that could otherwise be risked given the dangers of unusual hot condition or variable fuel quality. This appears to have been part of the problem the Germans suffered with the DB605A in raising boost from 1.3 to 1.42 ata and again from 1.8 to 1.98 ata.

Thermionic Vacuum Tubes (Valves if your British) were well up to the task. If operating at audio frequencies (not VHF/UHF) the valve could easily last 30,000-50,000 hours and handle 60G acceleration. They would be quite suitable. I don't think they'd actually be needed.

4 The Lambda Probe Oxygen Sensor is attributed to Robert Bosch GmbH during the late 1960s under the supervision of Dr. Günter Bauman. Im not sure if there was an equal sensor possible at the time, but I think there probably was. It would be useful for the kind of 2 stroke engines people were working on and indeed I see no problem incorporating it with the controls of the day. I'm just not sure if there was such a device in existence.

Personally I think had it not been for the jet engines 2 stroked turbo supercharged disels were the future. They stood a chance of actually producing 4000hp on 12-16 cylinder instead of 24,28 or 36.

Snowygrouch said:

In what way?

Other than being instantly programmable (i.e. being altered in 5seconds instead of needing a new cam ground) it does exactly the same thing.

It allows ignition timing, boost and fuel to be altered on the basis of inputs of temperture, pressure, and load. Most WW2 engines were NOT limited in performance
by virtue of over simplified control systems. They were limited by all the fundamental physics that wasnt yet sorted out, mostly combustion system
understanding.

All the tiny tweaking of transient performance is meaningless in a virtually constant speed engine like aero-engines.

The engines in the lab in 1940 were instrumented to similar degrees as they are now, cylinder pressure, EGT, exhaust gas analysis, albeit very laboriously and without nice logging, but the kommandogerats (etc) were not fred flintstone items and although of course you`d chuck them in the bin now, throwing them away will only gain you convenience not performance.

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The lambda sensor was possible in 1930s. It is just a Nernst Cell, invented in 1899 by Walther Nernst. It is attributed to Robert Bosch in 1960 but they just industrialised it. The Lambda sensor was introduced mainly to control Catalytic converters but its ability to improve performance was soon appreciated. The cell would give an electrical output proportional to oxygen, using 1930s technology this could ve be displayed in a dial gauge but also amplified and used to drive a small servo motor, say 2.5W which could adjust the mixture In the carburettor/fuel system.

Conslaw said:

The original Saturn S-series was competitive when it first came out. When Honda redesigned the Civic for 1993 model year, the Saturn was a generation behind.

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I loved my 2000 Saturn SL2. It was this second generation that I preferred. Too bad it rusted out beneath the plastic panels.

Admiral Beez said:

I loved my 2000 Saturn SL2. It was this second generation that I preferred. Too bad it rusted out beneath the plastic panels.

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Whatever happened with using plastic panels? Not terribly popular.

gjs238 said:

Whatever happened with using plastic panels? Not terribly popular.

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I liked them. I was side swiped on a highway off ramp. Their car spun and wrecked itself in a ditch. My Saturn had a shattered side panel. I drove to the junkyard and with a few wingnuts swapped out the panel.

Admiral Beez said:

I liked them. I was side swiped on a highway off ramp. Their car spun and wrecked itself in a ditch. My Saturn had a shattered side panel. I drove to the junkyard and with a few wingnuts swapped out the panel.

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Are you referring to hardware or your friends?