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Niels

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Aug 15, 2019
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Thank You for an interesting read.
Around 39 kg per litre being a prechamber diesel and 240 gram per kWh.
As gasoline engine it will need around 22kg per litre at same power level and have same fuel consumption.
Diesels may have a place in military but not in homebuilts where you drive to airports in gas cars.
The limit is the heat load on piston top and this is only around half if you burn gasoline and use spark plugs.
Double power for same engine mass.
If the WAM is made as a gasser it will at least sell as well as the present thing.
 
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PMD

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Martensville SK
Around 39 kg per litre being a prechamber diesel and 240 gram per kWh.
Diesels are infinitely more suited to aircraft than spark ignition. First of all, there is no reason for a modern turbo diesel to be anywhere near as heavy as a gasoline engine. Yes, one possible limit is piston crown temperature, but there is no practical difference between gasoline spark ignition and diesel compression ignition at the same cylinder pressures (=same power). What limits gasoline is cylinder pressure before ignition (i.e. MAP) that causes detonation - whereas diesels have no such limit as there is simply nothing in the chamber to detonate until the injection events begin. Turbocharged diesel engines simply add fuel and air until you reach some kind of limit - either mechanical or thermal - and that can be considerably higher than in an SI design at same RPM.

The fuel is much less critical (you can burn pretty much ANY hydrocarbon instead of the critically narrow range of fuels and mixtures required by gasoline SI engines. The fuel has better energy density, but most of all much safer as far lower flash point. There are none of high altitude issues with ignition as there is no ignition system. There is one extremely large advantage to turbocharged CI diesels in aircraft: You can pretty much tailor the power "curve" to be what you like at the RPM you want. That means you can go direct drive if you like (as in SMA/Continental 5.0 litre) but what nobody has bothered to do (yet) is to build a flat rated engine that simply makes its takeoff power at the start of the takeoff roll RPM and stays there as RPM increases - in other words, no need for a constant speed prop.

Then there are the more subtle advantages that can make an aviation diesel as light as or lighter than a gasoline SI engine: two cycle operation. Diesels simply do that far better at lower RPM than SI. On top of that, instead of just saving the wasted extra strokes of a 4 cycle, you can also gain a great deal of efficiency by going to uniflow scavenging instead of counterflow. Finally, there are diesel designs that don't have the complexity, weight, cost and thermal losses of cylinder heads - an option that IS available in SI, but just works better due to BTE considerations with CI.

I have to admit that designs now in production and certified are way too heavy for what they accomplish. Converting an automotive diesel is a dead end weight and complexity wise. The more aviation designs (SMA and soon EPS) come much closer to gasoline SI power densities, but both suffer the extreme penalties of being 4 cycles. There are designs in the hardware and test stages that go beyond these early attempts.

Worth remembering that the most efficient engine EVER installed in an airframe was a diesel in the '30s (Junkers Jumo).

BTW: I drive to the airport in diesel powered cars and trucks...in case you didn't guess.
 

PMD

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Apr 11, 2015
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289
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Martensville SK
I still got one of these engine in my garage!
lovely engine..
I can't understand why the pre-chamber???? I have a 4-53 and a much newer 6V-92T - essentially the uniflow engines that set the standard for how the Wilksch engines are built. I could not even begin to imagine them being hobbled by not being DI.

BTW: that really is a nice looking installation. Do you know the installed weight and horsepower?
 

rv6ejguy

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Jun 26, 2012
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Location
Calgary, Alberta, Canada
Diesels are infinitely more suited to aircraft than spark ignition. First of all, there is no reason for a modern turbo diesel to be anywhere near as heavy as a gasoline engine. Yes, one possible limit is piston crown temperature, but there is no practical difference between gasoline spark ignition and diesel compression ignition at the same cylinder pressures (=same power). What limits gasoline is cylinder pressure before ignition (i.e. MAP) that causes detonation - whereas diesels have no such limit as there is simply nothing in the chamber to detonate until the injection events begin. Turbocharged diesel engines simply add fuel and air until you reach some kind of limit - either mechanical or thermal - and that can be considerably higher than in an SI design at same RPM.

The fuel is much less critical (you can burn pretty much ANY hydrocarbon instead of the critically narrow range of fuels and mixtures required by gasoline SI engines. The fuel has better energy density, but most of all much safer as far lower flash point. There are none of high altitude issues with ignition as there is no ignition system. There is one extremely large advantage to turbocharged CI diesels in aircraft: You can pretty much tailor the power "curve" to be what you like at the RPM you want. That means you can go direct drive if you like (as in SMA/Continental 5.0 litre) but what nobody has bothered to do (yet) is to build a flat rated engine that simply makes its takeoff power at the start of the takeoff roll RPM and stays there as RPM increases - in other words, no need for a constant speed prop.

Then there are the more subtle advantages that can make an aviation diesel as light as or lighter than a gasoline SI engine: two cycle operation. Diesels simply do that far better at lower RPM than SI. On top of that, instead of just saving the wasted extra strokes of a 4 cycle, you can also gain a great deal of efficiency by going to uniflow scavenging instead of counterflow. Finally, there are diesel designs that don't have the complexity, weight, cost and thermal losses of cylinder heads - an option that IS available in SI, but just works better due to BTE considerations with CI.

I have to admit that designs now in production and certified are way too heavy for what they accomplish. Converting an automotive diesel is a dead end weight and complexity wise. The more aviation designs (SMA and soon EPS) come much closer to gasoline SI power densities, but both suffer the extreme penalties of being 4 cycles. There are designs in the hardware and test stages that go beyond these early attempts.

Worth remembering that the most efficient engine EVER installed in an airframe was a diesel in the '30s (Junkers Jumo).

BTW: I drive to the airport in diesel powered cars and trucks...in case you didn't guess.
Diesels are pretty well always heavier than the same power turbo SI engines due to the heavier components required for the high CR.

Only mechanical strength practically limits SI engines for hp output, witness the nearly 1000hp/L output on the '80s era F1 engines We have plenty of OEM SI engines outputting 150+hp/L on crap 87 octane these days, some production engines are edging 250hp/L on pump fuel and over 300 on E85. DI has considerably improved that from the old days.

The high altitude issues with diesels include: poor relight capability, very high pressure ratios required and subsequent higher intercooling requirements and cold weather limitations with the fuel gelling.

Diesels have inferior hp output to SI engines at the same MAP.

Modern SI engines match the BSFC figures of light diesels.

EPS is currently dead in the water...

While lighter, 2 Stroke diesels generally have inferior BSFC figures to 4 stroke CI engines. The Jumo 205 is bested today in BSFC by several light aero diesels like the Austro AE series and matched by most others.

You might want to look at modern SI turbo aero engines like the Adept 320T for a state of the art comparison.
 
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PMD

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Martensville SK
Diesels are pretty well always heavier than the same power turbo SI engines due to the heavier components required for the high CR.
That is a fallacy. cylinder pressure under compression is much less than pressure after ignition. It was true in the dark ages of low pressure injection (when you could hear the diesel "knock") that heavier components were required to tolerate detonation, but that is simply no longer an issue with HPCR and rate shaping. You can not only eliminate spikes in the MEP curve, but you can also shape it to take better advantage of con rod/crank geometry during power stroke.

Only mechanical strength practically limits SI engines for hp output, witness the nearly 1000hp/L output on the '80s era F1 engines We have plenty of OEM SI engines outputting 150+hp/L on crap 87 octane these days, some production engines are edging 250hp/L on pump fuel and over 300 on E85. DI has considerably improved that from the old days.
Just take a look at a real world Lycosaurus TIO-540 at altitude. The thermal load even on that very modest power density engine needs vast quantities of excess gasoline to be poured through the engine to keep it from melting down. Yes, DI has done marvellous things to modern tech SI engines, but HPCR has done similar things for CI engines.

The high altitude issues with diesels include: poor relight capability, very high pressure ratios required and subsequent higher intercooling requirements and cold weather limitations with the fuel gelling.
Yes, poor restart was the nemesis of the SMA 305, but that was a relatively crude injection system. Modern HPCR engines start just as well at low temps as SI gassers. Similarly, ULSD and Jet A simply don't have gelling issues at even the lowest of temps encountered. Old D2 and McDonalds frier oil are indeed a different thing, but not something any SI engine could even THINK of burning. The high pressure ratios are not so much required as desired - to get the power at the RPM needed.

Diesels have inferior hp output to SI engines at the same MAP.[/quote} I can't say that is true or not, will have to research that. Point is: there is simply no detonation to be encountered - the nemesis of SI and gasoline.

Modern SI engines match the BSFC figures of light diesels.
The very best ENGINE (hardly representative of the whole industry) is Toyota's new Dynamic Force 2.0 that comes in at a record breaking 40%. Weichai Group last year introduced a commercial diesel engine certified to 50.26% BTE - blowing the SI out of the water by about the same margin as across the full span of manufacturers. Cathedrals do about 55% now, but I have seen experimental (retrofittable technology) diesels hit 70%. Your generalizations are out of date.

EPS is currently dead in the water...
which is very sad to hear. Hope that get their shyte together again.

While lighter, 2 Stroke diesels generally have inferior BSFC figures to 4 stroke CI engines. The Jumo 205 is bested today in BSFC by several light aero diesels like the Austro AE series and matched by most others.
Again, now outdated information (take a look at Achates Power's work with 2 cycle opposed piston engines). I admit to taking some license with the old Jumo, but just consider that they did what they did with injection systems about as sophisticated as a surface carbeuretor. Imagine such a design with modern HPCR.

You might want to look at modern SI turbo aero engines like the Adept 320T for a state of the art comparison.
I will, thanks for the reference.
 
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tspear

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Feb 12, 2014
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Outside Boston
@PMD

CR has a significant impact on weight. From my internet knowledge, diesel typically is 18 or 23 to 1; while most avgas engines are 7.5 or 8.5 to 1 with many auto engines pushing 10 to 1 and some going as high as 14 to 1. However, the auto engines with the high compression such as the Nissan variable compression VC-Turbo Engine only run the higher compression when the load is low. Not exactly something we have a lot of while flying.

In terms of fuel flow, that is a red herring. Due to fixed timing of magnetos, we use fuel to control the flame rate. If you install a modern electronic fuel and ignition system than you do not send excess fuel out the tail pipe.

Tim
 

rv6ejguy

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That is a fallacy. cylinder pressure under compression is much less than pressure after ignition. It was true in the dark ages of low pressure injection (when you could hear the diesel "knock") that heavier components were required to tolerate detonation, but that is simply no longer an issue with HPCR and rate shaping. You can not only eliminate spikes in the MEP curve, but you can also shape it to take better advantage of con rod/crank geometry during power stroke.

Just take a look at a real world Lycosaurus TIO-540 at altitude. The thermal load even on that very modest power density engine needs vast quantities of excess gasoline to be poured through the engine to keep it from melting down. Yes, DI has done marvellous things to modern tech SI engines, but HPCR has done similar things for CI engines.

Yes, poor restart was the nemesis of the SMA 305, but that was a relatively crude injection system. Modern HPCR engines start just as well at low temps as SI gassers. Similarly, ULSD and Jet A simply don't have gelling issues at even the lowest of temps encountered. Old D2 and McDonalds frier oil are indeed a different thing, but not something any SI engine could even THINK of burning. The high pressure ratios are not so much required as desired - to get the power at the RPM needed.
Ok, show me a modern diesel of the same hp which weighs less than a modern than a modern SI turbo engine. How could a diesel be lighter? They have larger/ thicker pistons, rods, wrist pins, cranks and blocks.

Let's compare modern aero diesels to modern liquid cooled SI engines like the 320T. Comparable cruise BSFC figures to diesels and much better power to weight ratios.

The diesel won't start above 18,000 feet (or even 12,000 in some cases) because the ambient pressure is too low.

The Austro AE300/330 is limited to -30C with Jet A and -5C with diesel.

Of course SI engines don't burn old fryer oil... I don't see your point here. We'll be filling up at airports in most cases with 100LL, mogas or maybe unleaded avgas like Swift 94 or 100.

Most turbine aircraft have fuel heaters just because you have pumping, gelling and vaporization issues below about -45C. Diesel aircraft will have the same issues.

Diesels certainly do require much higher pressure ratios to make the same power as SI turbo engines- about double in fact. You can watch my video here which explores this topic:
 

sotaro

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Feb 26, 2011
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San Francisco
Ross, what about the Mazda CI vs SI 4 cylinder. My understanding is that they are using the same compression ratio, 14:1. My question is more about the weight differential than anything else. It seems the SI has more power and the CI has a somewhat greater specific fuel consumption, but not much greater.

 

rv6ejguy

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I don't see any specs on BSFC here, but with the same CR, the SI is likely to have lower BSFC because the Otto cycle is more theoretically more efficient than the Diesel cycle. I don't see any weight specs here either.
 
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