36 replies to this topic

[Magoo]

...wondering if anyone has been following the PPC engine development work by Bengt Johansson and Lund University. Using gasoline in a diesel, they make some rather tall-ish claims regarding TE and ME. 

 

Some papers: 

 

 

http://www.sae.org/e...gtJohansson.pdf

 

http://www.lundunive...&postid=1665749

 

http://www.erc.wisc....3-Johansson.pdf

 

 

 

I know there are some very sharp diesel people lurking around here and would like to know what we think. 

[Greg Locock]

He's being a bit naughty emphasising IMEP efficiency. The brake efficiency is 48.5%, which is a good figure for an uncharged engine, and the emissions look rather good.

[gruntguru]

I'm sure researchers like to compare IMEP between engines so differences in mechanical efficiency can be ignored. This is combustion research after all.

 

I wonder if J Edlund is around? He is familiar with Johansson's work.

 

Edit. I also notice that one of their test engines, the Volvo D5 was run as a single cylinder (other 4 cyls inactive). BMEP results from that configuration would obviously be pessimistic.

Edited by gruntguru, 01 March 2014 - 09:17.

[bigleagueslider]

A BTE of 48% is good, but you must consider that this was acheived under lab conditions.  It is very difficult to control HCCI or PCCI combustion in a normal operating environemnt.

[mariner]

Very interesting - it sort of raises the question of how far HCCI has progessed recently - it was the "great new thing" under discussion about two years ago.

 

IIRC one of the big US nuclear labs ( Sandia?) with experience in nuclear fission was working on modelling the complex conditions in HCCI to see what i, if any, controls could be used to keep it working across a range of revs/loads.

[PJGD]

Industry is following this work and is moving ahead with the teachings of Bengt and Gautam Kalghatgi.  There are several papers out there in the literature now, and this article from Car & Driver although not very technical, is nevertheless pretty close to the mark.  In single and multi-cylinder form, the efficiency and emissions achievements of GDCI on regular pump gasoline are impressive.

 

 

As for the modeling work on HCCI, that is being done at Lawrence Livermore National Laboratory.

 

 

[gruntguru]

. . . . and this article from Car & Driver although not very technical . .

 and downright inaccurate:

 

" yields cylinder pressures that rise far more gently than those found in any diesel"

 

Diesel cylinder pressures can be made to rise as gently as desired, by regulating injection rate. HCCI, PPC and GDCI all have faster pressure rise than typical Diesels. Diesel efficiency would be higher if cylinder pressure rose more rapidly at TDC - approaching the Otto cycle where all heat is added at TDC.

[Kelpiecross]

What happens if you run a typical car-size diesel on petrol? Does it blow up? not run at all? run roughly? etc. etc.? It must have been tried many times both intentionally and unintentionally. I read once that the pressure rise with petrol-in-a-diesel would be far too great - but as GG says - surely the pressure rise can be controlled by the rate of petrol injection?

[Greg Locock]

Kills the injector pump.

[gruntguru]

Yes many diesel fuel system components rely on the lubricity of diesel fuel. Those issues can be overcome but the underlying problem is cetane number. High octane fuels tend to have a low cetane number and vice versa. Cetane number is a measure of "ignitability". Low cetane fuels like petrol have a large "ignition delay" if used in a diesel engine. When fuel injection commences there is a small delay before ignition occurs. With a long ignition delay there will be a substantial quantity of unburned fuel present - in near stoichiometric proportions - before ignition occurs and a detonation eventwill occur as all this unburned fuel burns suddenly (diesel knock). The same phenomenon occurs when a diesel engine is cold.

 

PPC using petrol avoids diesel knock by careful preparation of the early injection, dilution with EGR and appropriate CR for the fuel octane number.

[Kelpiecross]

Yes many diesel fuel system components rely on the lubricity of diesel fuel. Those issues can be overcome but the underlying problem is cetane number. High octane fuels tend to have a low cetane number and vice versa. Cetane number is a measure of "ignitability". Low cetane fuels like petrol have a large "ignition delay" if used in a diesel engine. When fuel injection commences there is a small delay before ignition occurs. With a long ignition delay there will be a substantial quantity of unburned fuel present - in near stoichiometric proportions - before ignition occurs and a detonation eventwill occur as all this unburned fuel burns suddenly (diesel knock). The same phenomenon occurs when a diesel engine is cold.
 
PPC using petrol avoids diesel knock by careful preparation of the early injection, dilution with EGR and appropriate CR for the fuel octane number.

Thank you for that GG. I did know that this was the "official" reason why a diesel should not run on petrol (- apart from buggering the injector pump etc.) But as often seems to happen, in real life things do not really go as predicted. I had seen (years ago) a diesel car accidently filled with petrol apparently running better than it did on diesel. This video shows a similar result with the blue diesel Escort seemingly running better on petrol for half an hour or so before (presumably) the injector pump dies. Possibly it ran OK because of the diesel remaining in the tank modified the cetane/octane levels.

These new ideas of HCCI, PPC etc. seem like very complex solutions to the problem of running a diesel on petrol. And I personally am not sure just why people want to do HCCI, PPC etc. - just because petrol is a bit cheaper than diesel at the present?

This video, forgot to put it in:

Edited by Kelpiecross, 04 March 2014 - 05:02.

[Greg Locock]

In cold weather we used to put some petrol into the diesel at the farm, but i don't know why.

[Kelpiecross]

In cold weather we used to put some petrol into the diesel at the farm, but i don't know why.

Years ago in the bush I used to see people tipping a capful of petrol into the air intake of big bulldozers to help with cold starts in the morning - they swore by it. Conventional wisdom would imply that it probably shouldn't help the starting.

[Catalina Park]

Diesel turns to jelly in cold conditions. We run on "alpine" diesel here in winter to help with the cold starts. Once a diesel is running it warms the fuel tank with the returned fuel.
Tipping a little petrol in the air intake would be a lot different to trying to inject it into the cylinder. It would be like using ether as a starting agent. 

[gruntguru]

These new ideas of HCCI, PPC etc. seem like very complex solutions to the problem of running a diesel on petrol. And I personally am not sure just why people want to do HCCI, PPC etc. - just because petrol is a bit cheaper than diesel at the present?

No. Its because these combustion strategies have the potential for significantly increased efficiency.

[indigoid]

How does this compare with LPG-augmented diesels? They seem to be semi-popular in some segments of the offroader crowd

[gruntguru]

My experience with bolt-on LPG fumigation is usually poor efficiency and emissions. Performance is great but engine reliabilty is inversely proportional to power increase and duty cycle at WOT.

[Kelpiecross]

Diesel turns to jelly in cold conditions. We run on "alpine" diesel here in winter to help with the cold starts. Once a diesel is running it warms the fuel tank with the returned fuel.
Tipping a little petrol in the air intake would be a lot different to trying to inject it into the cylinder. It would be like using ether as a starting agent.

I think the point I was trying to make was that, from the previous discussion of the low cetane rating of petrol, it is a little surprising that the petrol will ignite at all - especially in the enormous cold cast-iron mass of a bulldozer engine on a cold winter's morning.

Judging by the performance of the diesel Escort in the above video - a research project into petrol (or a petrol/diesel mixture) fuelling of a diesel engine would be interesting. And before anybody says it - a diesel with its injector pump etc. modified to handle petrol. Such a project, I suspect, would be more relevant to practical use than HCCI/PPC etc.

Not that I have anything against HCCI/PPC etc. research - any such original research is useful and interesting.

[bigleagueslider]

HCCI gives high BTE due to its near constant volume combustion.  But it is very difficult to accurately control in real world driving conditions.  And HCCI combustion also creates very high cycle pressures and produces significant amounts of combustion noise.

[gruntguru]

Judging by the performance of the diesel Escort in the above video - a research project into petrol (or a petrol/diesel mixture) fuelling of a diesel engine would be interesting. And before anybody says it - a diesel with its injector pump etc. modified to handle petrol. Such a project, I suspect, would be more relevant to practical use than HCCI/PPC etc.

Why? Diesels run better and more efficiently on diesel fuel.

 

The research you suggest has been done - many decades ago and many times over.

[Kelpiecross]

Why? Diesels run better and more efficiently on diesel fuel.
 
The research you suggest has been done - many decades ago and many times over.

Obviously - but I think worthy of re-examination. You can never assume that something unexpected could be discovered.

[gruntguru]

I think you will find that such things are examined daily.

 

[Kelpiecross]

I think you will find that such things are examined daily.

Yes - you are probably right.

However one thing I would like to see examined is the high RPM performance of a petrol or petrol/diesel fuelled diesel. The high-end RPM performance of diesels seems to be limited by the relatively slow rate of burning of the diesel fuel droplets (or so they tell me)- I would think that a diesel engine that was mainly (or entirely) fuelled by petrol may have no RPM limitations due to burning speeds and may have similar high RPM performance to a petrol engine.

[gruntguru]

Can't find any supporting data but I don't think diesel fuel burns slower than petrol. Combustion rate in a diesel is limited by injection rate and mixing rate (of fuel/air)

[Kelpiecross]

Can't find any supporting data but I don't think diesel fuel burns slower than petrol. Combustion rate in a diesel is limited by injection rate and mixing rate (of fuel/air)

I did think that it was "conventional wisdom" (not that you can always rely on "conventional wisdom") that the various well-known characteristics of diesels such as low max. RPM, sooty exhaust, the need for much excess air etc. were all related to the fact that diesel fuel droplets have a much slower rate of burning than the more-easily vapourised petrol.

[gruntguru]

Nah, the real problem is you have to get those last droplets in - through the cloud of burnt mixture to find some oxygen.

[Kelpiecross]

Nah, the real problem is you have to get those last droplets in - through the cloud of burnt mixture to find some oxygen.

Which is the point of this little debate - surely the petrol vapour would have a much greater ability to "find" some oxygen than a diesel droplet?

[bigleagueslider]

Can't find any supporting data but I don't think diesel fuel burns slower than petrol. Combustion rate in a diesel is limited by injection rate and mixing rate (of fuel/air)

If you are considering the constant-volume type of combustion that occurs with HCCI, then there is no real difference between diesel or gasoline fuels. The constant-volume combustion that occurs with HCCI is virtually instantaneous. And it's this nearly instantaneous combustion and rapid heat release rate that makes HCCI so efficient.

[gruntguru]

Which is the point of this little debate - surely the petrol vapour would have a much greater ability to "find" some oxygen than a diesel droplet?

Not if you think about it. The fuel issuing from the injector is following earlier droplets into a cloud of combustion products - no oxygen left. No easier for a gas to penetrate than a liquid. (Probably harder).

 

As for "petrol vapour", in a diesel engine the fuel spray using petrol would contain less vapour than for diesel fuel. The latent heat of vapourisation for petrol is 50 - 100% higher than diesel. (Petrol 380 - 500 kJ/kg, Diesel 250 kJ/kg)

[Kelpiecross]

Not if you think about it. The fuel issuing from the injector is following earlier droplets into a cloud of combustion products - no oxygen left. No easier for a gas to penetrate than a liquid. (Probably harder).
 
As for "petrol vapour", in a diesel engine the fuel spray using petrol would contain less vapour than for diesel fuel. The latent heat of vapourisation for petrol is 50 - 100% higher than diesel. (Petrol 380 - 500 kJ/kg, Diesel 250 kJ/kg)

Are you saying that diesel evaporates more easily than petrol? The ease of evaporation of a liquid (and that is what we are talking about in an engine) is directly related to its boiling point/vapour pressure - not so much related to their latent heats of evaporation. Clearly petrol has a much lower BP/VP than diesel. Put a bowl of each out in the sun - I think you can probably rely on the petrol evaporating first.

Just what the relation between latent heat of evaporation and ease of evaporation is I am sure. I see in the list of latent heats that ethanol has much the same latent heat as lead - and I suspect the lead would evaporate much more slowly than ethanol.

[Kelpiecross]

If you are considering the constant-volume type of combustion that occurs with HCCI, then there is no real difference between diesel or gasoline fuels. The constant-volume combustion that occurs with HCCI is virtually instantaneous. And it's this nearly instantaneous combustion and rapid heat release rate that makes HCCI so efficient.

BLS - slightly off the subject - but you mention "constant-volume combustion - it would not be difficult to devise an engine design that did allow all the combustion to take place at constant volume (such as one of the many "cam" engines that inventors love). If you do a few rough calculations based on the peak combustion temperatures and exhaust temp of an SI engine - you will see that it implies an effective expansion ratio of 6 or 7:1 - when the engine has a nominal ER of 9 or 10:1. Probably some of this anomaly is due to the exhaust valve opening before BDC - but some is probably due to the fact that the combustion is nothing like at constant volume.
There may be an possibility that "constant-volume" engines could be made that would be more fuel-efficient that conventional engines - especially with diesels and their slower rate of combustion.

Edited by Kelpiecross, 10 March 2014 - 05:03.

[gruntguru]

Are you saying that diesel evaporates more easily than petrol? The ease of evaporation of a liquid (and that is what we are talking about in an engine) is directly related to its boiling point/vapour pressure - not so much related to their latent heats of evaporation. Clearly petrol has a much lower BP/VP than diesel. Put a bowl of each out in the sun - I think you can probably rely on the petrol evaporating first.

Just what the relation between latent heat of evaporation and ease of evaporation is I am sure. I see in the list of latent heats that ethanol has much the same latent heat as lead - and I suspect the lead would evaporate much more slowly than ethanol.

I think you will find the combustion chamber of a Diesel engine at TDC is a little different to "out in the sun".

[Kelpiecross]

I think you will find the combustion chamber of a Diesel engine at TDC is a little different to "out in the sun".

Just to make it very clear - are you saying that, when diesel fuel or petrol are injected into the combustion chamber of a diesel engine at TDC, the diesel fuel would evaporate more readily to form a vapour/gas than the petrol would?

[gruntguru]

Yes.

 

OTOH rapid evaporation might not be the optimal result. Droplets will penetrate further into the chamber (than a gas) before losing their kinetic energy.

[bigleagueslider]

Yes.

 

OTOH rapid evaporation might not be the optimal result. Droplets will penetrate further into the chamber (than a gas) before losing their kinetic energy.

The mean droplet size and dispersion are critical factors with DI diesel injector nozzels. The fuel droplets only combust at their surface where there is the proper mix of fuel vapor and oxygen. Large spray droplets have enough inertia to penetrate through the chamber space and inpinge on the piston bowl surface, resulting in quenching.

[Kelpiecross]

Yes.

We may have to agree to disagree on this one.

[gruntguru]

Yes.