RE: Diesel engines torque it up
Discussion
speedy_thrills said:
Would it be unfair to say that combustion in a compression engine is done in a more consistent manner than that of a spark ignition engine due to the ability to run at chemically “lean” (the amount of oxygen in the chamber exceeds that needed to combust the chemical fuel) charge mixture? (Assuming appropriate and even fuel dispersion throughout the cylinder)
It's your last assumption that screws it up unfortunately - the cylinder charge of injected diesel is not homogenous though with injector pumps they're getting better and better atomisation with 2000 bar+ injection pressures.
If it was ideal, lean, complete combustion then diesel engines wouldn't produce all those carbon particulates - rather the carbon would be consumed in the cylinder and come out as CO2.
nel said:
Oh yes, a more finely divided spray would ignite and burn much faster. If you look at the curve of cylinder pressure for a direct-injection diesel, not a lot happens for a significant while immediately after injection, while the relatively large droplets vaporise and mix with the air, then there's a sudden peak as it all goes off at once. With the improvement in atomisation and mixing from an air-blast injector, it'd start to burn as soon as it was injected.
Extremely finely divided combustible materials can be quite interesting when dispersed in air
Not sure about this improvement in atomisation issue - the ignition of the charge in a diesel engine is initiated by the increasing pressure, so all the diesel injected goes bang at much the same time. Hence the distinctive knocking noise of a diesel engine compared to the progressive, flame-front burn away from the spark-plug in a petrol engine.
If you improve the atomisation of the injected diesel, you will improve the efficiency of the resultant explosion, but I do not believe that it will "start to burn" any sooner - the same critical pressure regime still has to be reached.
Firstly, the reason a diesel is more efficient is because the amount of fuel delivered controls the power, rather than throttling the air and controlling fuel delivery as in a petrol engine. So yes, at part throttle diesel is more efficient. The higher compression ratio in a diesel also ensures greater fuel efficiency.
Improving atomisation, by higher injection pressures and smaller injector holes, gives better air/fuel mixing for rapid combustion and good torque output. In short, better atomisation will reduce the ignition delay period (the time period before a notable change in cylinder pressure). The suddden rise in cylinder pressure is reduced by pre-injection, the resulting pre-combustion heats the cylinder before he main injection and gives a smoother, quieter rise in pressure.
gommo said:
Firstly, the reason a diesel is more efficient is because the amount of fuel delivered controls the power, rather than throttling the air and controlling fuel delivery as in a petrol engine. So yes, at part throttle diesel is more efficient. The higher compression ratio in a diesel also ensures greater fuel efficiency.
Another great efficiency advantage of the diesel engine comes from the lower combustion temperatures - they lose far less energy as heat compared to a petrol engine. That's why it takes bloody ages to get warm in diseasel in winter....
Not at all! The lower the combustion temperature the lower the efficiency. You want it to burn as hot as possible. Hence the interest in ceramic combustion chamber materials, the funky alloys used for gas turbine blades, etc. The efficiency gains of a diesel come from the higher compression and the lower pumping losses.
Hi,
Several years ago, I saw the Documntary with Roby Coltrane about Rudolf Diesel. I rememebr Roby saying that the idea came to Diesel from something called "Malayan Fire machine" - a contraption apparently consisting of a Bamboo "cylinder" and a wooden "Piston", with dry leaves which ignited when the piston was pushe down and the Air heated.
I can't find any mention of it on the network, and my friends call me a Fibber.
Can anyone point me to a reference to this Malayan fire machine?( A search will bring many "Fire machine guns at..." in Malaya).
Shalom
Several years ago, I saw the Documntary with Roby Coltrane about Rudolf Diesel. I rememebr Roby saying that the idea came to Diesel from something called "Malayan Fire machine" - a contraption apparently consisting of a Bamboo "cylinder" and a wooden "Piston", with dry leaves which ignited when the piston was pushe down and the Air heated.
I can't find any mention of it on the network, and my friends call me a Fibber.
Can anyone point me to a reference to this Malayan fire machine?( A search will bring many "Fire machine guns at..." in Malaya).
Shalom
As far as I can see there are several factors,
1) diesel has a higher chemical energy content than petrol and can run lower air to fuel ratios (say 40:1 to idle)
2) There are no pumping losses without a throttle (or for the semantic, you no longer get those crated by the throttle)
3) The compression ratio does help, but it also brings increased heat losses (by raising the peak combustion temperatre and hence the temperature gradient to the cylinder walls)
Another thing not to be forgotten is the techology on modern diesel engines tends to be way in excess of that on their petrol equivilants as manufacturers struggle to reach emissions requirements. The chances are your diesel has anti-knock facilities (using pre-injection) variable geometry turbos or anti-lag (effective using post-injection) etc etc how many ordinary saloon car petrol enines do you know with that?
1) diesel has a higher chemical energy content than petrol and can run lower air to fuel ratios (say 40:1 to idle)
2) There are no pumping losses without a throttle (or for the semantic, you no longer get those crated by the throttle)
3) The compression ratio does help, but it also brings increased heat losses (by raising the peak combustion temperatre and hence the temperature gradient to the cylinder walls)
Another thing not to be forgotten is the techology on modern diesel engines tends to be way in excess of that on their petrol equivilants as manufacturers struggle to reach emissions requirements. The chances are your diesel has anti-knock facilities (using pre-injection) variable geometry turbos or anti-lag (effective using post-injection) etc etc how many ordinary saloon car petrol enines do you know with that?
shalom said:
Hi,
Several years ago, I saw the Documntary with Roby Coltrane about Rudolf Diesel. I rememebr Roby saying that the idea came to Diesel from something called "Malayan Fire machine" - a contraption apparently consisting of a Bamboo "cylinder" and a wooden "Piston", with dry leaves which ignited when the piston was pushe down and the Air heated.
I can't find any mention of it on the network, and my friends call me a Fibber.
Can anyone point me to a reference to this Malayan fire machine?( A search will bring many "Fire machine guns at..." in Malaya).
Shalom
Search for "fire piston".
Not to be outdone the petrol engine may yet fight back.
An interesting develpoment by a US technology company has enabled petrol cars to operate in open choke mode thus getting rid of one of the biggest losses in a petrol engine.
It also increases combustion efficency.
it works by decomposing some of the incoming fuel supply to a stream of Hydrogen and CO. The hydrogen can ignite over a large air fuel ratio range and ignites the petrol air mix.
Details here:
www.theautochannel.com/news/2004/06/11/199554.html
They reckon that it would give an increase of about 15 to 20% in economy. I reckon that it would depend on the engine and the usage profile.
Andy
An interesting develpoment by a US technology company has enabled petrol cars to operate in open choke mode thus getting rid of one of the biggest losses in a petrol engine.
It also increases combustion efficency.
it works by decomposing some of the incoming fuel supply to a stream of Hydrogen and CO. The hydrogen can ignite over a large air fuel ratio range and ignites the petrol air mix.
Details here:
www.theautochannel.com/news/2004/06/11/199554.html
They reckon that it would give an increase of about 15 to 20% in economy. I reckon that it would depend on the engine and the usage profile.
Andy
[quote=T&Ws Dad]'Diesel's new engine, which his wife convinced him to name the engine after himself...'
You need to get some English lessons!
[/quote]
Rock on, good effort resuscitating an ancient thread for that insightful comment
The sentence works if you admit an implied 'of' before the 'which' anyway.
You need to get some English lessons!
[/quote]
Rock on, good effort resuscitating an ancient thread for that insightful comment
The sentence works if you admit an implied 'of' before the 'which' anyway.
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