I have a question for you diesel combustion engineers and theorists...
An acquaintance mentioned something about diesel engines having a theoretical "minimum" displacement-per-cylinder size before diesel thermodynamic efficiency goes down the toilet.

Said differently: a hypothetical 1liter engine with 1 cylinder should be more efficient than a 1L 4cyl engine ... (assuming same "square" bore/stroke ratio, aspiriation and injection technology)

My question is: at what size cylinder is this "theoretical" minimum supposed to be?
Or, how "small" a cylinder can be expected to be used for a small diesel engine?

Keith

 

Thermodynamic efficiency is usually that portion of heat energy that is used to push the piston down vs what is absorbed into the cylinder wall, piston top and head. Friction losses wouldn't be considered by definition. You'd have to figure out how much energy is absorbed. Since more energy is absorbed into a cold combustion chamber than a warm one it also is dependent on temperature. This is why running an engine without a thermostat-cold will rob your mileage. The hotter you can run the engine the better for efficiency but other problems crop up at higher temps.

The absorbtion will also be greater with a higher surface area on the walls, piston etc... therefore there will be a minimum displacement size before the engine will function efficiently if at all.

The variables however are probably to many to give any better answer though.

Edwin

 

Interesting question I can't answer but, I also remember hearing about a "theoretical" maximum RPM a diesel can turn based on the speed of the combustion/expansion of the fuel.

Maybe the two are related.

Inquiring minds want to know.

 

Those are some interesting questions. When I did R&D at GM, we never looked at that aspect. The 1L 4 cyl vs 1L 1 cyl is relitively easy to answer. The 4 cyl will have more friction from 4 pistons and sets of valves and the 1 cyl will have less friction. Therefore the 1 cyl will be more fuel efficient, as long as the combustion chamber designs are equal. The engines are most fuel efficient at aprox 65-75% load. Designing an eng size for fuel efficiency is more getting the power to weight ratio in the band of the most efficient range of the eng, then reduce weight and friction, etc. Our diesels are pressureized and change that theory some because along with pressurizing the air intake you add more fuel and keep the combustion in the more fuel efficient band. Hope this helps

 

It's helpful to consider this in terms of ratios.

One thing is the ratio of displacement to surface area. The more surface area subject to heat loss for a given displacement, the less efficient it will be.

Another ratio is swept area to displacement. This is the surface area that the piston covers as it travels throughout its stroke.

For a given volume, the most efficient container is a sphere. It has the lowest surface area for a given volume. Thus, less area for heat to be lost.

So ideally, you'd have the fewest cylinders, and they would be nearly square. You wouldn't want a super oversquare or undersquare design.

So why don't we see large 2, 3, or 4 cylinder engines? Why no 5.7L Twin engines? There are a number of reasons why efficiency is sacrificed in favor of more cylinders for a smaller displacement. An engine with many small cylinders will breathe better and make more HP, though it may be less thermally efficient. It will have much lighter internals, rev to higher rpm and therefore be able to process more air/fuel and therefore make more power.

Compare a 3.0L V-12 to a 3.0L four-cyl and you will see how this plays out. I've done it, playing with the bore, stroke, and allowable valve sizes, etc. The V-12 will make MUCH more power. The smaller bore will have more desirable flame propagation characteristics, etc.


IMHO, any serious effort at efficiency should incorporate thermal barrier coatings wherever feasible.

jlh

 

Here is the smallest diesel. Hard to believe but .020 cu in. model airplane motor. These diesel 2 strokes pull much larger props for a given size than their "gas" counterpart. they've been around for a long time and are very fuel efficient. yes they blow black smoke and smell just like our trucks. They even have a bit of "knock" in minature haha.

http://www.davisdieseldevelopment.com/home.htm


bentwings

 

Ok, that is just COOL. If my R/C truck had an OS .12CV, (for which they make a conversion) I'd order that up tomorrow!

 

I understand that, but giving that the "shape" of the cylinder remains square(bore dia = stroke length) there is "supposedly" an effect that comes into play as an engine cylinder is reduced in size.

I think he said something to the effect that because of such a small mass of air to be squeezed that the avail "heat" on compression is lost much quicker than a bigger cyl with its bigger charge of air...
I don't quite understand this effect as it seems to me that it should be a "linear" (EDIT: "CONSTANT")effect.
Maybe someone much more versed in thermodynamics could explain that..
I can't quite look this topic up on "SNOPES.com" to find out if it is "Bravo-Sierra", so DTR gets the first shot!


On those "TINY" RC engines:
I remember those little model airplane diesel engines... They always had a variable compression bolt on the top to "fine tune" their running. I wonder what their compression ratios were for those tiny ones...
A working scale model of a WWII german JUMO diesel radial engine would be to die for!!!!

I admit I have never run a model airplane diesel engine myself. I had enough trouble keeping reliable glow engines running in my all too frequently doomed R/C planes.

Keith

 

If you're thinking in terms of specific fuel consumption, I would commend to your attention the superlongstroke designs currently used in many/most direct drive marine engines. In addition to the surface area-to-swept volume considerations already discussed, the superlongstrokes are designed to provide the greatest possible expansion ratio.

Rusty

 

A lot of the model plane engines are technicaly diesels even though they burn nitromethane fuel. A Diesel is an engine which uses compression for it's ignition heat rather than a spark plug. Glow plug engines qualify as diesels no matter what fuel they use.

Edwin

 

I have a 1950 John Deere R model diesel tractor that has a 416 cubic inch two cylinder on it (5 3/4 inch bore, 8 inch stroke). Has the two cylinder pony motor to start it too. The R started use in 1949 and set the world record for fuel economy. I left it set from September last year till just about two weeks ago and never touched it. I figured the battery would be dead and neither would start. After setting all that time, the pony fired up after it turned over about a dozen times and when I threw the levers to start the big engine, I turned it over decompressed until the oil pressure built up and released the decompress lever. It fired up and ran like it was never shut off. There is a lot to be said for simplicity and big bores.

https://www.dieseltruckresource.com/...cat/500/page/1

 

Man! Those big marine engines are cool!
When I first saw it, I looked at the SFC figures for that huge marine diesel that keeps getting posted here and elsewhere. They get some UNBELIEVABLE specific fuel consumption numbers! But they turn So SLOOOOOWWWWW!!!!!! (can you say "MEGA-TORQUE"?) They also burn some fuel we could mix gravel with and spread on our roadways!!!

This topic first came to my attention some time ago when an acquaintance was saying that a particular little tiny one-lung Yanmar engine he had was getting real close to the "theoretical" minimum size displacement for this diesel engine design. That, for a progressively smaller scaled engine design(all things being equal), it would get increasingly hard to get it to "ignite" on injection as the compressed air mass would eventually not be near as hot to reliably start compared to a larger cylinder(requiring an increase in compr ratio). And at the same time the thermo efficiency of the tiny cylinder would be so far down that it wouldn't be as efficient even when warmed up and running, all because of this same "effect".

This topic came up again this week when I mentioned I was still seeking a small kubota powered generator for my household emergency backup power needs.
He discounted the tiny, multi-cylinders of the Kubota approach (because of this "effect")in favor of going to one larger, single-cylindered engine for his off-grid power approach (I think he must really like concrete!).

Keith

 

I'm looking for the same thing since I use heating oil which is just died diesel fuel. I want to hook it to the tank and let it eat.

 

According to my Mechanical Engineers' Handbook (2nd Ed., Myer Kutz editor), Chapter 59.3.2, pages 1817-1818:

The traditional simplified model for [thermal efficiency] of the compression ignition (CI) engine is the air standard Diesel cycle, but the air standard dual cycle is more representative of most modern diesel engines. . . For the air standard dual cycle, it can be shown that

thermal efficiency = 1-[(1/CR)^(k-1)]*((rp*rv^k)-1)/((rp-1)+(k*rp*(rv-1)))

where

CR = compression ratio
k = ratio of specific heats of working fluid (air, assumed constant)
rp = ratio of maximum pressure to the pressure at the beginning of combustion process
rv = ratio of volume at the end of the combustion process to the volume and the beginning of the combustion process (TDC)

This equation indicates that thermal efficiency is a function of compression ratio and load. Granted this equation uses some assumptions, but the results are qualitatively correct.

For those that care, the assumptions are:
1) no intake or exhaust process
2) isentropic compression and espansion
3) combustion occurs initially at constant volume, then the remainder occurs at constant pressure
4) constant volume heat rejection
5) air is the sole working fluid and has constant k value

How's that for an explaination?

Seriously, it appears that assuming the small engine was exactly to scale, the thermal efficiency shouldn't change that much. The compression ratios would be identical, the k value is constant, and although the pressures and volumes associated with the combustion process would be different their ratios should be the same.

I'm sure the fuel efficiency would change, though, if for no other reason than there's only so little fuel you can pump into the cylinder. I imagine the smaller the engine, the more prone it would be to smoking from over fueling.

Regarding the temperature of the compressed air, since air is considered an ideal gas, that small volume would reach the same temps as the larger cylinder, there just isn't as much heat energy in the small volume. The heat would dissipate faster, but how long is any piston +/- 5° from TDC, when pre-combustion temps are highest? 2000 rev/min * 360°/rev * 1min/60sec = 12000°/sec, which works out to .00083 seconds to sweep out that 10° arc. Even if it only idled at 200 rpm, that's still only .0083 to sweep those 10°. From -5° to TDC to +5° is only ~0.4% of the piston's total stroke, so the pressure won't vary much and neither will temperature.

Well, I'm starting to get a headache. Maybe I'll think about it more this weekend. Then again, maybe not. The forecast here is calling for sun and 70°, with occasional periods of heavy beer drinking.

 

Actually rv above is the expansion ratio, as opposed to CR - the compression ratio. Since the energy generated by combustion is extracted and converted to mechanical work on the expansion stroke, expansion ratio plays a critical part in thermal efficiency and brake specific fuel consumption.

As alluded to in my post above regarding the superlongstroke 2-cycle marine engines, compression ratio and expansion ratio are not necessarily the same - on these uniflow 2-cycles, the expansion ratio can be much greater than the effective compression ratio if the exhaust valve closing is delayed on the compression stroke - same principle as the Miller or Atkinson cycle on a 4-cycle engine.

Rusty