Daft Diesel Engine Question

Twister_Ken

Twister_Ken

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Think I understand what the throttle on a petrol engine does - lets in more fuel and air so the bangs get bigger and the whole shooting match spins round faster.

But on a diesel there's always too much diesel being delivered to the injector (coz some of it goes back through a return line to the tank), so what does opening the throttle do?
 
Some diesels as I understand have a wide open throttle for air and control power by restricting the fuel supply. Some I think reduce the air flow with a throttle also makes starting easier but there may be other reasons.
This is because a diesel engine is compression ignition ie it is happy with a very lean mixture. THe ignition is timed by the injection of fuel.
In a petrol engine the fuel is sucked in with the air the ignition is by the timed spark. If the fuel air mixture was lean it would ignite by compression at any time in the stroke and usually before the piston is ready to go down causing destructive preignition (deonation).

The timing of the injection of fuel in the diesel is made more accurate by having a spring loaded valve in the injector which only opens at very high pressures. '000s of PSI. (ie much greater than the cylinder pressure) Fuel is bled from the injector to keep the fuel cool not vapourised (which would compress).
Otherwise I am not sure why fuel is returned to the tank.
Perhaps someone can explain more olewill On reflection I really doin't know much about it all.
 
[ QUOTE ]
Think I understand what the throttle on a petrol engine does - lets in more fuel and air so the bangs get bigger and the whole shooting match spins round faster.
...........................................
With a carburettor:-
Lets more air in (by opening a valve in the air intake) and that sucks more petrol in, so the "bangs" get bigger and the engine goes faster

With petrol injection:-

Has a throttle position sensor which notices the throttle is opening, sends its signal to the ECU which makes the injectors put more fuel in and advances the ignition timing.
.......................................................
But on a diesel there's always too much diesel being delivered to the injector (coz some of it goes back through a return line to the tank), so what does opening the throttle do?

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The throttle controls the injector pump. Open the throttle and more fuel is injected. The engine speeds up and sucks more air in which makes the engine speed up even more. The increase in engine speed makes the injector pump advance the timing a bit which speeds the engine up a bit more. When the engine gets enough air to get the diesel burnt most efficiently or the governor kicks in to limit the revs to a pre-set maximum the engine stops speeding up.

The injector pump has to be kept full of diesel at all times ( it lubricates and cools the pump too usually) the lift pump always has to deliver more fuel to the the injector pump than the injector pump can pump through the injectors at full throttle - otherwise it will run out of fuel to inject while it waits for the engine to speed up and deliver more fuel to the pump. Any excess fuel is returned to the tank.

Common rail diesels use petrol injection developed electronics to give a greater range of control over the whole process.

Simple really..... /forums/images/graemlins/wink.gif

ps for some clarification for William_H. - diesels usually have no "throttle" on the air inlet tract, as it would restrict airflow into the engine - and restrict power. That's why you get a vacuum pump on a diesel vehicle to give you servo-assisted brakes - there is very little vacuum in the inlet manifold.
 
In a diesel engine the throttle doesn't directly influence the fuel delivery.Instead it controls the centrifugal regulator by only allowing it to open a certain amount.The injection pump would always deliver the same amount of fuel as its stroke is constant, the quantity of injected fuel being determined by a spiral port in the pump body wich is rotated by the governor that ,as I said before ,has been set by the throttle control.This applies to jerk pumps,the more usual ones.
 
But on a diesel there's always too much diesel being delivered to the injector (coz some of it goes back through a return line to the tank), so what does opening the throttle do?

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There are basically two injection systems....Standard type with one pump for each injector and Common rail where a fuel rail is pressurised to a high pressure and the injectors are opened either mechanically or electronically .

The standard system has a lift pump which delivers pump to the injector pump. This is usually a diaphragm pump driven off the camshaft and is a spring loaded diaphragm operated by a cam in the engine. It pumps purely on demand ie the delevery is governed by demand and the spring compression.

The lift pump delivers to the high pressure fuel pump which will be either a rotary type or in line . The rotary pump is sometimes just a single pump with a distribution system which delivers each spurt from the pump to each cylinder in correct sequence. Sometimes these pumps also contain their own transfer pump but still need the lift pump to opersate happily.

The in line pump has one pump cylinder for each injector/cylinder and each pump delivers the correct metered amount.

The fuel goes from the pumps to the injectors where it is injected through very fine holes at the tip of the injector. Only the correct amount is delivered to the injector however as the injector has operating surfaces which need to be lubricated then there is a small controlled amount of leakage and this is what appears at the spill connection and either returns to the fuel tank or on some engines is fed back into the lift pump suction line.

The fuel pump also has controlled leakage for lubrication and as on many engines this pump is mounted in the crankcase then this fuel leaks into the engine oil hence the need to frequently change the oil. This applies especially to Yanmar Bukh and many Volvo engines with the Bosh system or similar.

The common rail system has been developed to obtain better emmission figures as it lends itself to give better control of start and end of injection. In this system there is a common fuel rail on the engine which is pressurised at around 5,000 psi by a high pressure pump. Each injector is connected to this rail and the injector has a control valve in the head which is triggered electronically . The engine management box sends signals to the injectors so that they deliver the precise amount of fuel with the precise start and end of injection.

This system is new to small engines however was quite common many years ago on certain ships engine where the controls were totally mechanical and one had total control of the injection period unlike a jerk pump where the start of injection is constant and the end of injection is determined by the amount of fuel being injected hence helix position.

Unlike petrol engines the speed lever does not operate directly on the fuel pumps and there is NO throttle in the manifold.

The engine gets as much air as it can sook !

Adjusting the speed lever adjusts the governor setting and it is the governor output which adjust the fuel pumps and the amount is controlled to keep smoke etc to a minimum. With more modern common rail engines these controls are also part of the function of the ecm box which is monitoring exhaust gasses etc and so adjusting fuel amount and timing to keep emissions within limits.

Just a basic guide !!!!

WHo else here enjoyed Doxford Engines with common rail systems?????? Maybe I should be in a museum ..

/forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif
 
I thik you will find that common railsystems have MUCH higher pressures than mechanical systems, in the order of 10x.
These pressures allow for much better atomisation of the spray, hence the better efficiency/economy. ( and the higher boost pressures, as all the road ones are turbo)
I am curious about the leakage into the sump comment. The only time I had that (Perkins 4-108 with CAV dpa pump) It turned the oil to jelly and I had to rebuild the engine.
Andrew
BTW, both my road diesels have no lift pump other than that in the injector pump (old tech. engines) I am not sure I would want a C Rail in my boat, too many elecy bits.-A
 
If common rail technology is applied to small yacht diesels, then I won't be first in the queue. My Ford car has just required a new injector, which failed with no warning. The new injector alone cost well over £300 and requires electronic kit to install it. I'm told that they are not servicable and Ford is at the cheap end of the cost spectrum. Boats really do not need this technology which caters for the demand for refinement in car engines.
 
I am curious about the leakage into the sump comment.
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This does not apply to perkins where the high pressure pump is mounted externally. However many old Volvo engines ,yanmar and Bukh to name a few used a bosch pump or similar which actually sat in the crankcase . This means that the fuel spill/leakage from the pump plungers goes into the sump oil. This leakage amount increases as the pumps become more worn:)

This means that the dipstick level is no measure of lube oil usage cos as fast as oil leaks past the rings diesel leaks into the sump. Slowly the lube oil becomes diluted with fuel and so must be changed regularly.
 
If common rail technology is applied to small yacht diesels
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It seems that emmision standards for RCD/CE are at a more tolerant level than for road vehicles and most engine builders can achieve the standard with good old jerk pumps...However common rail systems complete with associated problems are comming into the marine market /forums/images/graemlins/frown.gif
 
Not sure which car you have, but quite a few fords use Pug engines. I agree, I am used to being able to fix things myself. Modern cars are designed to stop you, forcing the job to an agent. I believe the online diognostics used by some have just been ruled uncompetative.
Andrew
 
BTW, both my road diesels have no lift pump other than that in the injector pump (old tech. engines) I am not sure I would want a C Rail in my boat, too many elecy bits.-A
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The Mini max fuel pump will operate just using the transfer pump within the fuel pump however on a boat it is prudent to fit a lift pump as well where there may be along run or suction head. I think Sabre always use a lift pump with these???
 
the area these new systems fall down on is when a fault
occurs, when without a diagnostic program a repair is
outwith the scope of most people ,mechs included.
not a huge problem on the road but when at sea its a
major one , at sea simple is best
with regards operating pressures standard injectors
"crack" around 175bar, common rail runs around 330bar
 
The governor controls the flow(squirts) of fuel until the revs match your setting. In the car analagy the trottle controls speed not power. So when you desengage the drive the revs stay constant. An ungoverned petrol engine will rev wildly if you declutch without lifting the trottle. Your petrol lawn mower has a system like a diesel. As do small site generators.
Lew, thanks for the correction on C-Rail pressures. I read the fig. in the early days of c-r, did sound V. high.
Andrew
 
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This means that the dipstick level is no measure of lube oil usage cos as fast as oil leaks past the rings diesel leaks into the sump. Slowly the lube oil becomes diluted with fuel and so must be changed regularly.

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I have a six cylinder Lister with a minimec inline pump. The engine oil level never changes between services. This is due to diesel dilution of the engine oil. You check the oil level to make sure the level is not rising, which would indicate more dilution than normal is occurring. The lister manual says the engine will perform normally with up to 60% dilution. The service interval is every 250 hrs.
 
[ QUOTE ]
The fuel pump also has controlled leakage for lubrication and as on many engines this pump is mounted in the crankcase then this fuel leaks into the engine oil hence the need to frequently change the oil. This applies especially to Yanmar Bukh and many Volvo engines with the Bosh system or similar

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I have a Volvo 2003T. The oil always rises about 1/2 inch up the dip stick but never higher. If I measure the oil level immediately after stopping the engine, the oil level is correct. If I do it several hours later, it is 1/2 inch too high. I concluded that the flow back from the oild filter and galleries, caused this level rise.

HOWEVER - after a short time the oil seems to get very runny, and black, and I was woried that diesek was getting into the oil. Should I get the oil tested, and is there any cure to stopping the deisel leak?. I change the oil twice a year.
 
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