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Internal Combustion Engine Question
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Posted by: matin

06/29/2004, 03:59:25

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What is the effect of retarding and advancing the ignition in a spark ignition(SI) engine and the same for the diesel engine.

 







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Re: Internal Combustion Engine Smile
Re: Internal Combustion Engine -- matin Post Reply Top of thread Forum
Posted by: Morella

07/08/2004, 15:48:17

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Advancing ignition timing makes the spark happen sooner, retarding makes it happen later. That may sound pedantic, but it's a baseline for everything else involving timing. It gets more complicated from there.

To get maximum efficiency and power, the timing has to match the burn rate of the fuel at all RPMs. Too soon and you will get preignition, which not only robs you of power but can damage the engine. Too late and the piston is already going down by the time combustion occurs, so you lose power. As the engine turns faster, and the burn rate of the fuel is the same, the spark must occur sooner.

Older engines had two mechanisms in the distributors for dealing with this. One was a centrifugal (mechanical) advance, using counterweighs working against spring tension to change the position of the static assembly as relative to the rotating assembly. The other was a vacuum advance, which worked one of two ways, depending on how the hose was connected.

Straight manifold vacuum will advance the timing at idle, which makes the engine run a little cooler, but also increases emissions. This prevents hot start kickback by bringing in the extra advance after the engine has started. As vacuum increases, timing will increase. At WOT, when preignition is most likely to occur, there is no vacuum, so the timing is backed off.

Ported vacuum is an invention that is designed to decrease emissions. At idle, there is no vacuum advance. A port on the carb gives it more vacuum as the throttle is opened, until the point where vacuum drops off (near WOT), where there is no vacuum.

This gets more complicated because gasoline comes in many octane ratings. Lower octane fuels burn more rapidly, and so the timing needs to be retarded. High octane fuels have a smoother burn rate and you can get away with more advance. This is a generalization, but, up to a point, the more vacuum advance you can get away with, the more efficiency and power you will get.

There is one more variable, and that is the engine design itself, compression ratio and camshaft design. In general, a higher compression ratio means that you need higher octane fuel, or a retarded spark. Turbo(super)chargers also raise the manifold and cylinder pressures, having nearly the same effect as raising the compression.

Today's engines use computers to control spark and fuel. They have knock sensors that allow the engine to be taken as close to the knock point as possible for any octane fuel, and at all power settings. Older cars can benefit from a distributor recurve, where different mechanical advance springs and an adjustable vacuum advance are used to match the engine. In stock form, most high compression engines need premium gas and low compression engines can get by on the cheap stuff.

Hope this helps

-morella







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Re: Internal Combustion Engine
Re: Re: Internal Combustion Engine -- Morella Post Reply Top of thread Forum
Posted by: Cragyon
Bart
07/08/2004, 16:00:33

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Great post Morella!

Good read....







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Re: Internal Combustion Engine
Re: Internal Combustion Engine -- matin Post Reply Top of thread Forum
Posted by: pierre3737

07/01/2004, 23:27:13

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You want timing to produce maximum combustion pressure at about 20 degrees after TDC In a SI engine advancing will cause slightly more power but will cause detonation and difficult cranking of engine and also pinging under load which should reduce with use of a higher octane gasoline. If advanced to much you will melt the pistons. In a diesel your engine will sound much crisper(like broken dishes) when you accelerate and you will notice a considerable gain in acceleration. once again if you go to much you will burn pistons and your NOX emissions will increase as well as combustion temperature with a slight decrease in turbo boost pressure and reduced exhaust temperature. Retarding will do you no good but may even cause piston errosion on the diesels along with increased exhaust temperature and a drastic reduction in acceleration and power along with black exhaust smoke.

Hope this helps







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