Diesel vs miller cycle

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stanislavz

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Long story short. Diesels achieve~ 230-240 g/kw*hr bsfc in broad load range, where gasoline could achieve 300-330 g/kw*hr bsfc, in a narrower one. Where Prius engine could get it same or better as diesel.

Or like this one :

First and second generation Prius (Atkinson cycle) - 37% thermal efficiency.
Third generation Prius - 38% thermal efficiency.

Passenger car diesel engine (Diesel cycle) typical thermal efficiency ~ 34%.

Passenger car gasoline engine (Otto cycle) typical thermal efficiency ~ 28%.

And yes, you can put turbo on a Prius motor and get it from Atkinson to Miller cycle.

Cycle itself is just clever cam which does not close intake up to 70% of stroke is left. So we have less compression stroke, more expansion. And yes it is robbing power from engine too. So big and clumsy engine.

Same is one Diesel... And both of the do need a turbo to be viable in aviation uses.

Why not ? extra part to break ? need for efi which also over complicates all ?

On the other side we do have diesel with mechanical pump. Which adds some extra mass, but overall is a winner for slightly longer distances. Same as was suzuki conversion compared to two stroke rotax - yes, engines is heavier, but whole mass of load is lower for any longer mission.
 

rv6ejguy

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The 2018 and up Toyota Dynamic Force engine line is around 40% TE at high load near torque peak rpm. Given the lighter engine and similar BTU content per pound of fuel, that would seem like the way to go for aviation.

EFI is well proven to be very reliable and durable over the last 50 years and was the single most important technology in the automotive world to reduce maintenance, improve fuel economy, emissions and increase durability and reliability. Most certified CI aero engines use EFI today.
 

stanislavz

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The 2018 and up Toyota Dynamic Force engine line is around 40% TE at high load near torque peak rpm. Given the lighter engine and similar BTU content per pound of fuel, that would seem like the way to go for aviation.

EFI is well proven to be very reliable and durable over the last 50 years and was the single most important technology in the automotive world to reduce maintenance, improve fuel economy, emissions and increase durability and reliability. Most certified CI aero engines use EFI today.
Yes, but this one is with variable lift system, similar to used in Fiat multi-air engines. It is anything, but complex one. And it works for wide load range. On the other hand - we need only a custom camshaft and a turbo in our narrow rpm load range for aircraft uses.
 

PMD

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The 40% Toyota engine is a pretty significant step forward in the SI world, but over in the CI camp there is a lot of work going on as well. I know of one engine that has been run at around 70% thermal efficiency and the researcher claims he has a long way to go before reaching it theoretical limits.

BTW: if we think of the 2 cycle thermal efficiency limits in SI engines, it is worth noting that CI engines can be built with no such problems - as they have no aspirated charge to deal with. Recent work has brought the issue of oil loss through exhaust ports under control. Watch the 2 cycle diesels over the next little while - a LOT of development potential.

While such engines could easily be built with an all mechanical injection system, the potential benefits of very fine control of fuel timing, delivery, rate shaping and attendant turbocharger control to suit makes electronic control pretty much essential.
 
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trimtab

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I know of one engine that has been run at around 70% thermal efficiency and the researcher claims he has a long way to go before reaching it theoretical limits.
Thermal efficiency is just one problem to solve. Raising the compression ratio to 30 or more, insulating the cylinder and piston from heat loss, etc., make it possible. At the expense of weight, life, and especially NOx.

A 70% thermally efficient ICE is largely an academic exercise, unfortunately.

For a given compression ratio, the Otto wins in thermal efficiency.
 

PMD

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Thermal efficiency is just one problem to solve. Raising the compression ratio to 30 or more, insulating the cylinder and piston from heat loss, etc., make it possible. At the expense of weight, life, and especially NOx.

A 70% thermally efficient ICE is largely an academic exercise, unfortunately.

For a given compression ratio, the Otto wins in thermal efficiency.
Not merely academic, a running engine (based on a modified production engine). Wish I was free to speak more about it, and wish I knew more than the little slice of this I have been exposed to. The secret (and good NOx) is in reducing combustion temps. Have to stop there, but I can say that water plays a BIG part in it.
 
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dwalker

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I had a Miller cycle Mazda Millenia, wish I still did, it was a very fuel efficient car for its size, weight, and power.
 
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trimtab

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Not merely academic, a running engine (based on a modified production engine). Wish I was free to speak more about it, and wish I knew more than the little slice of this I have been exposed to. The secret (and good NOx) is in reducing combustion temps. Have to stop there, but I can say that water plays a BIG part in it.
I ran an engine using a Bosch diesel injector with water to 15kbar. The effect allowed enthalpy recovery, and the exhaust temps were carefully controlled (varying the supercritical water charge) allowed large compression increases in the engine from about 9:1 to 14:1 using a hexane fuel mix. The cylinder finally have up the ghost from the test, however (a small OEM industrial ICE), and the concept was proven. Then I found an ASME article from the 70's where essentially the same thing was done (conversion of the engine to an overexpanded Atkinson, higher compression, and the use of kerosene and heptane as a fuel, heating the water to supercritical temps and pressures using exhaust flow) to raise thermal efficiency from 20%'s to the upper 30%'s.

The fuel pump survived the use of water for many hours with a small amount of glycol added, and the injector would not have survived the erosion by the supercritical water much longer had the cylinder not cracked from the higher compression. The EGT's were maintained between saturation to around 400F by varying the injection volume by the Bosch pump.

I changed to a Delphi solenoid injector with a driver I built and it survived a little better in terms of erosion, but I had less control over volume...the minimum attainable volume was more than the maximum volume the small engine could use.
 

PMD

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20 or so years ago I had the experience of driving a Ford econoline running on 50% ethanol and 50% water - using catalytic igniters. Same guy who has developed the ultra-efficient diesel.

An M.E. prof I knew once tried an experiment of injecting water into a normally aspirated diesel (actually a John Deere farm tractor). It ran better and better as they added more and more water. In the end, he described it as pretty much a garden hose into the air inlet and it STILL kept running....better and better.

Anecdotal, but true. We still have a LOT to learn about what does and can go on when we burn hydrocarbons.
 

stanislavz

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For a given compression ratio, the Otto wins in thermal efficiency.
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Nope, the Atkinson one is first to win. But it lacks power density. But - being 20-30% more efficient - all that it takes is to add the turbo and convert it to miller one.
 

PMD

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Nope, the Atkinson one is first to win. But it lacks power density. But - being 20-30% more efficient - all that it takes is to add the turbo and convert it to miller one.
I think you are missing the point: when researchers can demonstrate nearly double the accepted level of small high speed diesel efficiency and show the way forward to further improvements much of the published data is now simply out of date. It doesn't seem to entail any change in the cycle, but exploiting the combustion process itself. I have no idea if the same could apply to SI engines at the Atkinson or Miller level of cycle structure - but that seems quite plausible.
 

stanislavz

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You can't be carrying a bunch of water in an airplane to make this scheme work so it's academic.
Exactly. But - it is not academic to grind a new camshaft or take motor from prius like car and convert it to carb/turbo.
 

dwalker

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You can't be carrying a bunch of water in an airplane to make this scheme work so it's academic.
I have been planning to use an water/alcohol injection system on my turbo rotary flight engine, but keep arguing with myself about the weight vs benefit. Most likely I will simply run it on E85.
 

stanislavz

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I have been planning to use an water/alcohol injection system on my turbo rotary flight engine, but keep arguing with myself about the weight vs benefit. Most likely I will simply run it on E85.
Or take non-turbo diesel and add 10% of lpg from camping bottle. For take-off and climb. Good for 20 more hp at least.. And ti will make performance of non turbo diesel from very sluggish to just sluggish :)
 

PMD

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You can't be carrying a bunch of water in an airplane to make this scheme work so it's academic.
Did not get to spend enough time on last visit to get fully updated, but the original work was with aqueous fuels (LONG ago) that lead to discoveries that I THINK are harvesting water from the exhaust. Carrying water might make sense if the net result was more range and/or power on the same fuel+water mass as "dry" flight. At 70% BTE it would of course depend on the dosing needed (and need to keep water from freezing). That is, IF supplementary water is still needed.
 
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