Propane injection for diesel.

"Thus they choose a compression ratio that really is too high for all other times, except for this worst case. This cuts efficiency as the engine is uselessly over compressing the air charge once it is warmed up. Oh if only someone could invent a variable compression ratio diesel engine!"

NONSENSE

The higher efficiency of ANY compression engine is why even cheap auto engines use high compression and a computer to time the spark for minimum pre ignition.The expansion of and use the burned fuel charge is what creates power, the more expansion the more Efficiency.

Variable compression ratio diesel engines have existed since at least the 1930's when diesel was used as aircraft engines to extend range and duration.

Either supercharging or turbocharging can vary the operating CR with ease.

A diesel with a very low compression ratio can be started by using an ALTERNATE fuel , to start.

This is done today with Ether injection to start large engines below -20F ,synthetic oil is required.

I met a fellow that was attempting to get diesel engines into the power range that small aircraft turbines now operate, 1 lb per hp at 300 to 500HP.

His concept was that the fuel was wasted by heating air after ignition , and his test engine has both supercharger (to be able to start easily) and turbo charger in operation.

Inlet pressure at the piston was over 1500lbs!!! And he was trying to go higher!!

Hope he finds US funding.

FF

 

In case of these diesel with "low compression" starting with ether or another light fuel before going to heavy oil, is there a spark or is it auto-ignition with a conventional diesel cycle ?

 

Fred,
You have not presented any evidence that shows what I said to be nonsense at all. Your original assertion about the diesel engine's efficiency stemming from its high compression is not supported with any objective facts or data. Show us how and why a compression ignited engine can benefit from a compression ratio that is far and above over what is needed to induce combustion at any given moment. You can't show that, because it's not the reality of the situation. The fact that so many others have worked to overcome this very engineering puzzle is of itself evidence that the puzzle exists.

You seem to pretend that there is no engineering 'problem' at all, and that more and more compression is all good. So why not go with 25:1, or 32:1 or 40:1? Do you think there are power and efficiency gains up there, if only the engine could be made sturdy enough?

This is all known, mature science, Fred. The ideal compression ratio for power and efficiency in a diesel engine is only 13.5-15:1, no higher. But such an engine will not start cold, thus the higher compression ratios.

All the schemes of variable compression tried so far are too costly or too complex for mainstream use, or reduce reliability to an unacceptable level. But none of these secondary factors is evidence that diesels would not benefit from a variable compression ratio scheme without these shortcomings.

Jimbo

 

Higher Compression does produce more torque with more fuel too. The reason for lower compression on many diesels is as an aid to starting. It is not that same to start a 10,000 hp diesel or fuel oil engine at 20 to 1 compression than 10 to 1. You open valve to lower compression to get rpms up, inject ether or some other fuel then close valves switch back regular fuel. As far as higher compression most new diesel forgo higher compression by using turbos. Same effect.

 

Diesels dont need high compression!!! funny how they love their turbos.

Ive never heard of 1500lbs intake pressures but tripe turbos can getup to 150, if the bottom end lasts.

I would have thought it a simple afair to make a cylinder with a piston arrangement in the top facing down that could be moved, like a small glo plug engine I had on a model airplane, it would screem hight RPm when the thumb screw on the top of the head was turned to increase compression ratio.

Maybe a deisel will run on low Ci once warmed up because of the energy to compress,--but the hight Ci increases efficiency and overcomes the extra load.

A turbo in the exhaust flow actually takes power from the engine inhibiting free flow of gases. But when the benefits of the compressor side of the turbo is taken into consideration it works out as a benefit as we all know.

 

Only up to a certain optimum point. Increasing the compression above this optimum point, whether you add more fuel or not, will not result in a further increase in torque at all. That point turns out to be ~13.5-15:1.

Sort of, kind of true, but the complete picture is more complex. Valve overlap causes every engine (except some really big ones) to spill some of its compression at very low engine speeds, like cranking speed. This can drop the effective compression ratio to below the minimum for a cold start. So reducing the mechanical ratio can reduce the 'breakdown' torque required to get the engine spinning initially, increasing cranking speed and the heat generated by a quicker succession of compression strokes.

Dual fuel engines are one way of addressing the 'too high' compression ratio of most diesel engines. It works well but is considered too costly and complex for most smaller engines like in the automotive field.

Turbochargers do nothing to help the engine start cold; all these compromises between mechanical compression ratio, camshaft profile (valve overlap) and cranking speed must sill be negotiated to retain cold starting capability.

Jimbo

 

Jimbo,
This is all so very interesting. The more I learn about internal combustion the more fascinating it becomes. Thank you.

 

the fact that diesels wouldn't' like high compression ratios is news to me too. (for performance that is) - and as stated high pressure super/turbo charged engines have very high comparable cylinder pressures. And generally raising compression ratio directly helps efficiency - on gas you are limited by pre-ignition and detonation.

It really is news to me if 15:1 would be ideal from efficiency stand point.

another big reason for diesel efficiency is the fact that it can run with lean ratios - in a gasoline engine on partial throttle positions the intake air is restricted and engine is pumping against choked intake.

Gasloine starts to ping and knock on high compression and lean situations - diesel is fine for both.

edit:
This study suggest that 15 indeed is close to being the best number

http://www.arpnjournals.com/jeas/research_papers/rp_2008/jeas_0408_88.pdf


however still supercharging isn't in line with that chart - I wonder if practical dimensions have to do with that - ie. with very high compression ratio your cylinder head and combustion chamber designs are forced to be far from ideal.

 

It is very possible that the shape and dimensions of an engine with variable compression ratios is quite poor.

Perhaps a side valve with poor volumetric efficiency?

The question is what relationship does the test engine have with any other current engine?

Varying the injection pressure ,a pre-combustion chamber , or an engine with more or less combustion chamber area , as well as better breathing might change all this dramatically.

After all IF any of these results are usefull to real engines,the engine test labs of EVERY engine builder worldwide are incompetant fools?

And have been blowing the bosses cash for decades?

FF

 

I know a little about diesels, I know Detroit Diesels fairly well so I will speak from that point of view. A non-aspirate 6-71 puts out 200 some hp, very easily and it can be boost with a turbo and aftercooler to about 430 hp. that is 1hp per Cu in. Not to bad. The bad news is the engines lifespan is cut tremendously when you double horsepower. The turbo unit has a lower compression. Going back to Non-turbo version, there is a high-compression 20:1 and low-compression 17:1. and many version in between. I don't know where 15:1 idea can from. But seems to me that is relate to using oversize turbo to putout more power, and lower emissions in idle, than an issue with diesel putting out more power at idle.

As far as propane injection, one of these days I am going to try it. Because I like longevity and fuel economy of my 200hp diesels, but on occasions I would like a short burst of power and perhaps propane is way to go for that. I know my engines can take 400hp for a little bit...

 

Remember that Detroit diesels and some others like EMD use a two-stroke cycle so the HP/Cu In numbers are a little skewed; these engines have twice the 'swept displacement' of a four stroke engine the same size.


The 15:1 number came from many years of research and published papers from the SAE and other engineering sources. Quite a few diesel tech books have referenced this number too, including the one I posted earlier.
Jimbo

 

The bad news is the engines lifespan is cut tremendously when you double horsepower.


The most realistic way to contemplate engine life is not with hours, but with fuel burned.

While it is true an engine burning 3X the fuel will probably have less than 1/3 the life of a more modest power requirement

A 6-71 at 1200rpm (Prime generator) has 20x the service life in hours from the sport fish 430 hp defuller, but is far closer when the actual total gallons consumed to rebuild is counted.

FF

 

True, that is why it is better to have 200hp engine, 1000 hours a year, and a 400hp 10 hours a years. Propane might do that.

 

Also notice that you could never run a diesel on 100 percent propane, the fuel properties are too different, it only add the extra power from the propane fuel burning in the excess O2 in the mix. I think it is questionable if there would be much of a actual efficiency increase, perhaps a bit. But you have to compare your total operating cost at the same power output, not just measuring diesel fuel use.

Also both gasoline and diesel fuels have much more energy per pound than propane, so you would have to carry much more fuel to get the same amount of power output over the same time interval. Plus hazard of a carrying around a high pressure tank of a very flammable gaseous fuel on a boat.

 

Absolutely right, I would only think of using as an emergency, like being chased by pirates or a hurricane. The propane would be stored outside for this use. I don't even use propane in kitchen on my boat. No cost efficient anyway just like nitrous on car.