Does anyone know how indirect injection works, please?
I know that the diesel fuel is injected into a chamber first, but how does it then get into the cylinder?
Also, why do some engines have this set-up rather than direct injection?
Thanks.
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Air is sucked (or forced by a turbocharger) into the engine by the induction stroke of the 4 stroke cycle, on an indirect injection engine the fuel is injected into the inlet tract so it enters the cylinder through the inlet valve with the air. On a direct injection engine the fuel is injected directly into the cylinder seperately from the air, the latter offers more control though is only really demonstrably offers a benefit when it comes to diesel engines.
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Chris
Indirect injection diesels use a small 'pre-combustion' chanber connected to the main combustion chamber. The fuel is injected into this. it was intended to slow the burn and reduce diesel knock. DI diesel do withut this, and inject directly into the main chanber. Improvements in injector design, increased fuel pressure etc have now minimised knock with the DI system.
JS
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John - quite right. Cheddar, I'm afraid you're wide of the mark. The principle of any diesel engine is that air (either naturally aspirated or blown) is compressed in the cylinder and pre-chamber if an IDI, and the fuel is injected into the resulting very hot air. Ignition occurs due to the compressive heat in the charge - hence the term compression ignition engine.
In an IDI, a small pre-chamber connects with the main combustion chamber via a relatively narrow passage. The advantage of this is that the rate of rise of cylinder pressure on ignition is limited by the restriction of the passage, thus reducing diesel knock. Other advantages of the IDI are that it is often easier to induce turbulence into a constant volume chamber, thus improving atomisation. The "mixture" in the pre chamber is also arranged to be richer than is required for final combustion immediately above the piston.
The downside of the IDI are the pumping losses caused by the narrow passage from the pre-chamber. Big diesels have always been direct injection to maximise efficiency, but developments such as pilot injection have allowed the rate of rise of cylinder pressure, and hence to some extent the knock to be limited. Small DI engines are now commonly available, but are in my view still somewhat rougher than a good IDI of equivalent size.
659.
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John - quite right. Cheddar, I'm afraid you're wide of the mark.
Rubbish, I simply gave a basic outline to somone who, from the way the question was put, clearly has little knowledge of this subject, furthermore not all indirect injection engines have or had a pre chamber.
Small DI engines are now commonly available, but are in my view still somewhat rougher than a good IDI of equivalent size.
You are I assume talking diesel only here. Early mass produced diesel DI car engines were less refined though more efficient than previous IDI diesel engines however contemporary Common Rail type diesel Di engines running extremely fine control of injector timing duration and volume are light years ahead of IDI in refinement efficiency and performance. On the otherhand as I indicated previously Di has yet to be shown to offer real benefits in respect of petrol engines.
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I'm glad someone has started this thread, because I have often been puzzled by the existence of IDI diesels.
The old traditional diesels I understand - air is compressed in the cylinder, fuel is injected into the combustion chamber at very high pressure at precisely the right moment, fuel is instantly vapourised and explodes, by product is the wellknown diesel knock.
So in an indirect injection engine, the air is compressed in the cylinder, and therefore in the interconnected chamber too? But the diesel is injected only into the chamber? So how does the resulting explosion manage to get down the small comunicating passage in time to continuing burning in the rest of the air in the combustion chamber itself? Or is that delay the whole point of the exercise?
How does the exhaust get swept out of the pre-chamber - does it have its own exhaust valve?
Doesn't the constricting effect of the connecting passageway cause problems at higher speed? Surely the delay will increase with speed?
Presumably the space at the top of the cylinder is smaller, to maintain the same high compression ratio, but now including the volume of the pre-chamber too? Or is CR now a different concept in these designs?
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Cheddar gave a good description of direct and indirect injection for a petrol engine.
Cliff Pope's questions indeed pose the questions which make the case for direct injection diesels.
The induced delay with the pre-chamber made for quieter burning with a simple injection system.
It is only with modern electonically controlled multiple injection pulses that the noise of a direct injection diesel can be controlled.Pumping losses are reduced and the nominal compression ratio is also reduced.
An early version, Perkins Prima, was rather noisy.Fitted to the Montego/Maestro.
--
I wasna fu but just had plenty.
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Thanks 659 You've added the detail to my basic description.
I agree on the small DI engines. Had a Fiests TDCi on rent a while back. Terrible diesel knock. When you picked up load it was like having Santa's elves all working on tinplate toys under the bonnet. This one didn't want to rev over 4k either. Nice car, shame about the engine......
JS
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So it all comes down to delaying combustion and reducing diesel-knock.
I read on an earlier thread that you can run an old Mercedes 5-cylinder IDI engine on straight vegetable oil (assuming the weather is warm enough) and that using this in DI engines will damage the injectors.
Is this because the delayed combustion in an IDI engine means that the harmful acids and deposits are produced away from the injectors?
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How does the exhaust get swept out of the pre-chamber - does it have its own exhaust valve?
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I'd imagine its such an energetic process, bieng linked into the combustion chamber, that the turbulent, expanding gases have no problem exiting via the usual exhaust valve. This is what: www-g.eng.cam.ac.uk/125/achievements/ricardo/ have to say "The Comet Mark III compression swirl chamber. The most famous of all diesel combustion chambers, this design embodied intense swirl with a reasonable rate of pressure rise and good fuel consumption. In its latest form it is still one of the most widely used chambers when, power rather than economy, is the prime consideration. It was the subject of a lengthy legal battle about infringement of Patent rights between Riccardo?s and Rootes? Bros. won by Ricardo?s who were represented by a young lawyer named Stafford Cripps (later to become Chancellor of the Exchequer). Many other engine techniques (stratfied charge etc) on this site dedicated to Sir Harry Ricardo.
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Nortones is right. There's no great problem with gases left in the precombustion chamber. The 'normal' exhaust valve in the main chamber suffices.
JS
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There's a simple description, written by Ivor Carroll, with diagrams at
www.dieselcar.com ,
Choose Articles/Diesel Power/ How Diesel Works
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Thanks Johns S & M. I see it now. I had an image in my mind of the precombustion chamber being more detached. It was someone's description of the "narrow communicating passage" I think that got me visualising it wrongly, like a kind of backwater or cul-de-sac.
Simple swirl chamber is not so very different from an ordinary traditional diesel anyway, isn't it? Haven't they always sometimes had very odd-shaped pistons, designed to trap or swirl the gases around? Unlike petrol pistons which are often flat?
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>>>> Di has yet to be shown to offer real benefits in respect of petrol engines.
Not really, DI petrol engines have the advantage of 10-15% better fuel consumption. Traditional port fuel injected petrol engines induct an homogenous mixture of fuel and air (usually 14.6:1 or thereabouts). This is because it is not possible to ignite much leaner mixtures using a spark and using this composition of fuel to air allows removal of most exhaust pollutants by the catalyst.
DI petrol engines can ignite a locally (in the region of the spaek plug) rich mixture (rich enough to be ignited by a spark) while keeping the overall fuel air ratio in the cylinder high (20:1 or higher) which lowers fuel consumption. This is achieved by situating the injector close to the spark plug and clever design of inlet passages and the piston crown. Disadvantages are the need to use sulphur free petrol and the requirement to run the engine rich periodically to purge the catalyst of NOx.
>>The old traditional diesels I understand - air is compressed in the cylinder, fuel is injected into the combustion chamber at very high pressure at precisely the right moment, fuel is instantly vapourised and explodes, by product is the well known diesel knock.
No the fuel does not explode and all burn instantaneously. Time is needed for the fuel to heat up, vapourise and mix with air. This is known as the ignition delay period which depends upon the Cetane rating of the fuel used and other factors such as the amount of air turbulence in the engine. For example vegetable oil has a lower Cetane rating than diesel oil is it?s ignition delay period is greater.
>>>So in an indirect injection engine, the air is compressed in the cylinder, and therefore in the interconnected chamber too? But the diesel is injected only into the chamber? So how does the resulting explosion manage to get down the small communicating passage in time to continuing burning in the rest of the air in the combustion chamber itself? Or is that delay the whole point of the exercise?
The initial explosion (known as the pre-mixed phase of combustion) occurs in the small pre chamber, the hot gases expand through the narrow passage which limits the rate of pressure rise in the cylinder and hence diesel knock.
Following on from this explosion, fuel is still being injected ( injection duration is controlled by throttle position) and burned depending upon how quickly the fuel can vapourise and mix with the available air (known as the diffusion burning phase). Because of the limited volume of air in the pre-chamber, most of this fuel is burned in the main chamber itself after partial combustion in the pre-chamber.
>>Doesn't the constricting effect of the connecting passageway cause problems at higher speed? Surely the delay will increase with speed?
No it doesn?t because the amount of air turbulence in the engine increases with speed, so the air and fuel mix faster and the ignition delay is reduced.
>>>I read on an earlier thread that you can run an old Mercedes 5-cylinder IDI engine on straight vegetable oil (assuming the weather is warm enough) and that using this in DI engines will damage the injectors.
Is this because the delayed combustion in an IDI engine means that the harmful acids and deposits are produced away from the injectors?
Not entirely, the viscosity of vegetable oil is higher than diesel so it forms relatively large fuel droplets. The high degree of air turbulence in the pre-chamber helps these to break up and burn more quickly and completely reducing diesel knock. DI electronically controlled injectors rely on correct diesel fuel to give correct spray pattern and injection volume which are critical on these engines.
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