Can someone please explain to me the why diesel engines do not use a spark but the heat of compression to ignite the fuel whereas petrol engines use a spark. Why do petrol engines need the spark if the fuel will ignite with a high enough compression ratio. I.e. why do petrol engines use spark and diesel do not.
I have trawled through this forum and others searching for an answer to this but have been unable to find one. Whilst looking around I read that petrol as a liquid does not burn, but evaporates and the vapour will ignite. Is this correct ?
Cheers guys.
Diesel / Petrol question.
Started by
marcus123
, Dec 17 2004 14:58
22 replies to this topic
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#2
JtP
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Posted 17 December 2004 - 18:48
Petrol will ingnite spontaneously in an IC engine, its just that the compression ratio of a car engine does not allow this to happen normally. In model aircraft F2D glow plug combat motors, it is normal to flush the motor with petrol after use to remove used fuel. They use 10% nitro methane/ 70% methanol/20% castor and the nitro can break down into nitric acid. They use pressure feed from a rubber bladder tank for a fuel system so the starting technique is allow some fuel into the engine, flick it a few times and then connect the glow plug. They can often fire while flicking from the petrol lying in the engine, unfortunately while not ready to get your finger out the way and it hurts.
#3
Xig
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Posted 17 December 2004 - 19:19
I'm not expert in disel nor petrol engines (actualy I'm not expert in any kind of enigne), but sometime ago I read the article here . It's not that technical, but is good enough for all we who don't worry (mainly because we don't understand) about what happens inside the engine. Hope it helps, while the experts in the forum post their answers 
#4
Pat Clarke
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Posted 17 December 2004 - 23:33
Usually (not always) petrol engines compress a fuel/air mixture. Adiabatic heat rise will cause the mixture to detonate spontaneously, and the engine will probably be damaged, hence petrol engines are limited to approximately 10~1 compression ratio.
Diesels, on the other hand, only compress air. The compression ratio is about twice what it is on a petrol engine (over 20~1) therefore the pressure is 4 times that seen in a petrol engine. The fuel is injected into the hot air by a timed pump or ECU triggered injector. This gives the ignition control needed to keep detonation under control.
It is important to understand that diesels always run at wide open throttle. The power output is not controlled by the amount of air being induced, but by the amount of fiel being injected.
No air throttling has several advantages. There is no loss of efficiency caused by energy being wasted generating a depression in the manifold, and diesels are particularly suitable for turbocharging as the turbo is not subject to the pressure fluctuations seen in a petrol induction system.
The high combustion pressures mean the diesel engine develops very high torque, and therefore lots of low speed power. This is a good thing because the very high pressures also require that the engine is built much stronger (heavier?) and needs to be fitted with a heavier flywheel to damp out the compression pulses. So, a diesel engine doesn't want to rev, but it doesn't have to as the power is developed at low RPM. Good fuel economy is an additional benefit of the low RPM.
The current crop of electronically controlled 'common rail' diesels have very high fuel pressures (up to 24,000psi) which gives clean running and low noise.
Emission controls on diesels make them possibly the only IC engines currently available that will meet European regulations, so although I am not a fan of diesels, I can see them getting even more acceptance in the near future.
Pat
Diesels, on the other hand, only compress air. The compression ratio is about twice what it is on a petrol engine (over 20~1) therefore the pressure is 4 times that seen in a petrol engine. The fuel is injected into the hot air by a timed pump or ECU triggered injector. This gives the ignition control needed to keep detonation under control.
It is important to understand that diesels always run at wide open throttle. The power output is not controlled by the amount of air being induced, but by the amount of fiel being injected.
No air throttling has several advantages. There is no loss of efficiency caused by energy being wasted generating a depression in the manifold, and diesels are particularly suitable for turbocharging as the turbo is not subject to the pressure fluctuations seen in a petrol induction system.
The high combustion pressures mean the diesel engine develops very high torque, and therefore lots of low speed power. This is a good thing because the very high pressures also require that the engine is built much stronger (heavier?) and needs to be fitted with a heavier flywheel to damp out the compression pulses. So, a diesel engine doesn't want to rev, but it doesn't have to as the power is developed at low RPM. Good fuel economy is an additional benefit of the low RPM.
The current crop of electronically controlled 'common rail' diesels have very high fuel pressures (up to 24,000psi) which gives clean running and low noise.
Emission controls on diesels make them possibly the only IC engines currently available that will meet European regulations, so although I am not a fan of diesels, I can see them getting even more acceptance in the near future.
Pat
#5
clSD139
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Posted 18 December 2004 - 04:55
If you keep yours eyes on the road (and not on other "participants of traffic") diesel can be fun! Older models even rev a bit (even higher than some petrol variants of by example the R5).
Indirect earlier self-burners are less noisy than industrial diesels, but use 20% more. Now you can find the dCI tag also on trucks, but renault doesn't has a model without turbo.
The golf SDI (sauger-diesel-injection) one of the few engines that are really maintainance free: no turbo and automatic adjusters. Of course: oil must be changed more and another weak point are individual pumps (instead of common-rail).
Al together Rudolf Diesel's simplifications are effective. Only to in favor of production costs or performance sparks are usefull.
dominickDS
Indirect earlier self-burners are less noisy than industrial diesels, but use 20% more. Now you can find the dCI tag also on trucks, but renault doesn't has a model without turbo.
The golf SDI (sauger-diesel-injection) one of the few engines that are really maintainance free: no turbo and automatic adjusters. Of course: oil must be changed more and another weak point are individual pumps (instead of common-rail).
Al together Rudolf Diesel's simplifications are effective. Only to in favor of production costs or performance sparks are usefull.
dominickDS
#6
clSD139
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Posted 19 December 2004 - 00:02
I wondered: the injection pressure on a indirect mechanical dieselpump is around 200 Bar. Direct injection of petrol is 150 Bar currently. Could it be possible to use these petrol injectors to ignite diesel?
#7
clSD139
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Posted 07 January 2005 - 19:34
Afther finding information about the flash point and auto-ignition I think it is possible to inject instead of spark petrol. But... HCCI (homogenous-charge compression-ignition) engines already exist!!! Will it be cleaner than current Direct Injection technologies? VW and Toyota worked together to get it to this level (especially low-engine-charge catalysator).
#8
johnyboy
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Posted 08 January 2005 - 23:06
Originally posted by clSD139
Afther finding information about the flash point and auto-ignition I think it is possible to inject instead of spark petrol. But... HCCI (homogenous-charge compression-ignition) engines already exist!!! Will it be cleaner than current Direct Injection technologies? VW and Toyota worked together to get it to this level (especially low-engine-charge catalysator).
For diesel HCCI combustion, the benefits are low NOx and particulate emissions but with a fuel economy penalty, for gasoline HCCI benefits are NOx and fuel economy. Honda had a two-stroke motorcycle in the market in the 90's operating HCCI combustion in some speed/load conditions.
J
#9
clSD139
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Posted 12 January 2005 - 20:49
I know about a real motorcycle on diesel, but a HCCI operation mode for commercial vehicles is new to me. Does it cure the higher consumption of a 2 stroke?
#10
Todd
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Posted 12 January 2005 - 21:05
Originally posted by clSD139
I wondered: the injection pressure on a indirect mechanical dieselpump is around 200 Bar. Direct injection of petrol is 150 Bar currently. Could it be possible to use these petrol injectors to ignite diesel?
The injector doesn't ignite the diesel. The compression does. This is determined by calculating the swept volume of the cylinder combined with the volume of the combustion chamber divided by the volume of the combustion chamber alone. The high pressure injectors allow the fuel to be injected into the combustion chamber when the air in the chamber is already compressed, but it doesn't raise the compression ratio meaningfully with the small volume of fuel being injected.
#11
clSD139
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Posted 13 January 2005 - 15:51
That's why I think it's extraordinary some geek managed it to build a diesel engine that revs to 7000 rpm! Injection for swirling, distributing, mixing, and igniting together at near TDC is an achievement on itself, and surely at higher rpm.
#12
J. Edlund
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Posted 14 January 2005 - 04:48
Unlike what some people think diesels develope similar specific torque outputs as a gasoline engine.
Diesels operate with the autoignition principle, if the temperature gets high enough, an air/fuel mixture will ignite. In a diesel it does so with first almost explosion rate (which is casing the knocking sound of a diesel) and then with what is called a diffusion flame.
Diesel engines, using direct injection operate at high fuel pressures, around 2000 bar, this result in a better air/fuel mixture in the cylinder with smaller fueldrops. Diesels don't use and throttle and are runned with air excess and in general one can say that the more air excess one got the cleaner and more efficient the engine will be. This plus that diesels aren't instable and can't suffer from engine knock is why they responds so well on turbocharging. Perhaps you wonder what I mean with instable; a gasoline engine is lambda controlled, more air = more fuel = more exhaust energy = higher boost and the circle goes round like that until something breaks. This also makes it hard to find a turbocharger for a gasoline engine that works well on all engine speeds, modern boost control have of course make the "width" of the turbocharger better.
A turbocharger on a diesel is today almost like bread and butter, this since a turbocharger improve both emission, weight-to-power ratio and fuel consumption.
A diesel engine starts inject the fuel a couple of degrees before TDC, it then continues to inject fuel during the combustion. When injection stops, combustion pressure will go down.
A problem with diesel engines are their high combustion pressures which result in a high engine weight. If you take a look on a heavy truck you will notice that they doesn't aluminum in the block or head, this is actually because that aluminum would fail because of the pressure, not instant of course but after time.
The slow engine speeds together with often high boost levels makes the engines to produce a lot of torque and this can be a problem too. At least it was for the diesels built for racing. As we all know, if we want performance, we want power, not torque, so a diesel with the same power output as a gasoline engine produce more torque as the power is given at a lower speed. Except for causing problems with a heavy engine it also requires a very strong and thereby heavy gearbox. One way to solve this is by designing a high speed diesel.
HCCI is a combustion method that works by auto ignition like the diesel but inject the fuel like the gasoline engine. The problem is to get the fuel to autoignite when we want it to ignite. That is also why they are still experimantal. Today HCCI mostly works under certain conditions, there are also problems with getteing complete combustions at the same time and controlling the autoignition.
And no, it isn't possible to iject diesel with injectors from a direct injection gasoline engine. The diesel injects fuel duing combustion and since we can have combustion pressures of 150 bars not much fuel would be injected wouldn't it! 1000-2000 bar is the way to go, with common rail or unit injectors (high pressure pump combined with injector).
Diesels operate with the autoignition principle, if the temperature gets high enough, an air/fuel mixture will ignite. In a diesel it does so with first almost explosion rate (which is casing the knocking sound of a diesel) and then with what is called a diffusion flame.
Diesel engines, using direct injection operate at high fuel pressures, around 2000 bar, this result in a better air/fuel mixture in the cylinder with smaller fueldrops. Diesels don't use and throttle and are runned with air excess and in general one can say that the more air excess one got the cleaner and more efficient the engine will be. This plus that diesels aren't instable and can't suffer from engine knock is why they responds so well on turbocharging. Perhaps you wonder what I mean with instable; a gasoline engine is lambda controlled, more air = more fuel = more exhaust energy = higher boost and the circle goes round like that until something breaks. This also makes it hard to find a turbocharger for a gasoline engine that works well on all engine speeds, modern boost control have of course make the "width" of the turbocharger better.
A turbocharger on a diesel is today almost like bread and butter, this since a turbocharger improve both emission, weight-to-power ratio and fuel consumption.
A diesel engine starts inject the fuel a couple of degrees before TDC, it then continues to inject fuel during the combustion. When injection stops, combustion pressure will go down.
A problem with diesel engines are their high combustion pressures which result in a high engine weight. If you take a look on a heavy truck you will notice that they doesn't aluminum in the block or head, this is actually because that aluminum would fail because of the pressure, not instant of course but after time.
The slow engine speeds together with often high boost levels makes the engines to produce a lot of torque and this can be a problem too. At least it was for the diesels built for racing. As we all know, if we want performance, we want power, not torque, so a diesel with the same power output as a gasoline engine produce more torque as the power is given at a lower speed. Except for causing problems with a heavy engine it also requires a very strong and thereby heavy gearbox. One way to solve this is by designing a high speed diesel.
HCCI is a combustion method that works by auto ignition like the diesel but inject the fuel like the gasoline engine. The problem is to get the fuel to autoignite when we want it to ignite. That is also why they are still experimantal. Today HCCI mostly works under certain conditions, there are also problems with getteing complete combustions at the same time and controlling the autoignition.
And no, it isn't possible to iject diesel with injectors from a direct injection gasoline engine. The diesel injects fuel duing combustion and since we can have combustion pressures of 150 bars not much fuel would be injected wouldn't it! 1000-2000 bar is the way to go, with common rail or unit injectors (high pressure pump combined with injector).
#13
johnyboy
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Posted 14 January 2005 - 21:35
In answer to a couple of previous replies:
The HCCI bike, the Honda ARC 250 (Active Radical Combustion, I believe), doesn't improve fuel economy so much as improve emissions. And although this technology could be used to improve FE, the use of direct injection would be more effective.
Modern LD diesel engine's do produce more torque than their gasoline counterparts, but I think the situation is further exaggerated by the fact that diesel engines are tuned to have best volumetric efficiency (breathing) and therefore torque at low RPM. The result is a car that feels much torquier (sp.).
J
The HCCI bike, the Honda ARC 250 (Active Radical Combustion, I believe), doesn't improve fuel economy so much as improve emissions. And although this technology could be used to improve FE, the use of direct injection would be more effective.
Modern LD diesel engine's do produce more torque than their gasoline counterparts, but I think the situation is further exaggerated by the fact that diesel engines are tuned to have best volumetric efficiency (breathing) and therefore torque at low RPM. The result is a car that feels much torquier (sp.).
J
#14
clSD139
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Posted 16 January 2005 - 02:20
I know it's a waste but if petrol was injected with higher pressure and adapted temperature would it work? Though HCCI seems also very enlightning.
#15
J. Edlund
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Posted 16 January 2005 - 18:28
Originally posted by johnyboy
In answer to a couple of previous replies:
The HCCI bike, the Honda ARC 250 (Active Radical Combustion, I believe), doesn't improve fuel economy so much as improve emissions. And although this technology could be used to improve FE, the use of direct injection would be more effective.
Modern LD diesel engine's do produce more torque than their gasoline counterparts, but I think the situation is further exaggerated by the fact that diesel engines are tuned to have best volumetric efficiency (breathing) and therefore torque at low RPM. The result is a car that feels much torquier (sp.).
J
"Ricardo has calculated that a 4.6 litre turbocharged LMP1 CI engine could be developed to produce at 1500 rpm as much as 1400 Nm torque (and thus 300 bhp). That figure equates to a bmep of 38.29 bar, which is produced pumping a plenum pressure of 3.0 bar. If one considers it in terms of bmep per bar plenum pressure, one can see at 12.76 bar it is the sort of figure one could obtained from the equivalent gasoline engine"
Race Engine Technology no. 005
In other words this mean that a diesel doesn't produce more torque than a gasoline engine, the high torque levels produced by diesels are just a result of high boost pressure.
Diesel engines found in diesel trucks can produce some very high torque levels, typically a 12 litre engine delivers 3000 Nm or close to it. But the point that easily can be forgotten is that these engines typically have plenum pressures of 3 bar absolute, and 1000 Nm from a 12 litre gasoline engine shouldn't be such a big deal.
#16
marcus123
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Posted 17 January 2005 - 09:17
Thanks for your answers, but what is about the relative properties of petrol fuel and diesel fuel that results in one being injected towards the end of the compression cycle and burning spontaneosuly, and one where the air fuel mixture is compressed and ignited with a spark or to put it another way, why wouldn’t you build an HCCI engine to use petrol or an ingnition engine to use diesel.
Cheers
Cheers
#17
clSD139
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Posted 18 January 2005 - 09:26
Diesel power is created to exploit compression-efficiency. The air is heated more so knock or autoingition can occur. It wasn't the inventors intention to get rid of sparks but they weren't necessary anymore if fuel is added adequeatly.
If the explosive propellant (and petrol is easier to activate then diesel) is mixed before the compression, it is more difficult to control. HCCI is a part solution, sometimes.
Spark diesel requires heavier sparks, it has a higher flash-point. Also the specific energy is less. And it can be used better for high-compression application.
If the explosive propellant (and petrol is easier to activate then diesel) is mixed before the compression, it is more difficult to control. HCCI is a part solution, sometimes.
Spark diesel requires heavier sparks, it has a higher flash-point. Also the specific energy is less. And it can be used better for high-compression application.
#18
J. Edlund
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Posted 18 January 2005 - 20:27
Both diesel fuel and gasoline have similar energy contents, about 42 vs. 43 MJ/kg, but the diesel is slightly denser, .84 vs. .74 kg/dm^3.
Othervise the fuels are quite different. Diesel boils at 180-360 degC while gasoline (premium, lead free) boils at 25-215 degC. Diesel has a higher flash point and is therefore safer.
Diesel is made from heavier hydrocarbons than gasoline and they have some different properties when it comes to ignition and also boiling points as I mentioned earlier. In a diesel the cetane number is an important factor, the cetane number depends on how easy/fast the fuel is ignited. With gasoline engines we instead talk about the octane number which depends on how easy the fuel will detonate, that is being ignited by the combustion heat before the flame front has arrived.
A good fuel for a Spark Ignited engine will have a high octane number, also resulting in a low cetane number while the good diesel fuel will have a high cetane number (fastgoing diesels are requiring higher cetane values) also resulting in a low octane number. So a diesel fuel can't be used in a gasoline engine, it will result in detonation while the Compression Ignition engine (diesel) won't be able to ignite a SI engine fuel properly. For example, if one would like to run a diesel on ethanol for example a very high compression ratio would be required, or a fuel addetive which helps it to ignite.
The advantage with the diesel cycle is that the engine can't suffer from detonation, the fuel can however burn too fast or too slow, which allows high compression ratios and boost levels. The lack of throtteling is the other advantage.
Othervise the fuels are quite different. Diesel boils at 180-360 degC while gasoline (premium, lead free) boils at 25-215 degC. Diesel has a higher flash point and is therefore safer.
Diesel is made from heavier hydrocarbons than gasoline and they have some different properties when it comes to ignition and also boiling points as I mentioned earlier. In a diesel the cetane number is an important factor, the cetane number depends on how easy/fast the fuel is ignited. With gasoline engines we instead talk about the octane number which depends on how easy the fuel will detonate, that is being ignited by the combustion heat before the flame front has arrived.
A good fuel for a Spark Ignited engine will have a high octane number, also resulting in a low cetane number while the good diesel fuel will have a high cetane number (fastgoing diesels are requiring higher cetane values) also resulting in a low octane number. So a diesel fuel can't be used in a gasoline engine, it will result in detonation while the Compression Ignition engine (diesel) won't be able to ignite a SI engine fuel properly. For example, if one would like to run a diesel on ethanol for example a very high compression ratio would be required, or a fuel addetive which helps it to ignite.
The advantage with the diesel cycle is that the engine can't suffer from detonation, the fuel can however burn too fast or too slow, which allows high compression ratios and boost levels. The lack of throtteling is the other advantage.
#19
Cociani
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Posted 18 January 2005 - 20:54
A mechanic friend of mine showed me a 1980's Datsun,(Nissan) 4 cylinder naturally aspirated diesel engine out of a light pickup which had a throttle body and a butterfly operated throttle. Why would they have added an un-necisary part like that? It was apparently a 2.5L capacity.
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#20
bob sacramento
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Posted 18 January 2005 - 23:05
Thanks Mr Edlund, I am slowly getting there.
So if i am understanding correctly, diesel has better ignition properties, higher cetane, lower octane and gasoline has better detonation properties, lower cetane and higher octane.
Why does the fuel with the heaver hydrocarbons have better ignition properties/ worse detonation properties that the one with the lighter hydrocardbons. Is it do with how well the fuel mixes or vapourises.
I have read loads of stuff on this board and others on octane number but dont really know anything about cetane numbers,
Cheers
Marcus
So if i am understanding correctly, diesel has better ignition properties, higher cetane, lower octane and gasoline has better detonation properties, lower cetane and higher octane.
Why does the fuel with the heaver hydrocarbons have better ignition properties/ worse detonation properties that the one with the lighter hydrocardbons. Is it do with how well the fuel mixes or vapourises.
I have read loads of stuff on this board and others on octane number but dont really know anything about cetane numbers,
Cheers
Marcus
#21
AS110
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Posted 18 January 2005 - 23:25
Originally posted by Cociani
A mechanic friend of mine showed me a 1980's Datsun,(Nissan) 4 cylinder naturally aspirated diesel engine out of a light pickup which had a throttle body and a butterfly operated throttle. Why would they have added an un-necisary part like that? It was apparently a 2.5L capacity.
They had a vacuum govenor - of course a diesel has no vacuum,so they provide some.
#22
clSD139
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Posted 19 January 2005 - 01:04
So, a 2-stroke diesel has less cetane but not enough octane for spark-ignition?
I supose a diesel can't run lean or somthing. Or it always does?
I supose a diesel can't run lean or somthing. Or it always does?
#23
crono33
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Posted 21 January 2005 - 17:12
i have been looking for documents or informations about emissions in diesel or petrol engines and comparisons
cannot find much (i havent searched a lot i have to admit) but what i have found relates only to old diesel engines. i know new generation diesel engines are much better but would be interesting to see where these 2 engines are emissionwise.
gm
cannot find much (i havent searched a lot i have to admit) but what i have found relates only to old diesel engines. i know new generation diesel engines are much better but would be interesting to see where these 2 engines are emissionwise.
gm
