Why?

[Sector-9]

Just been out replacing contacts on my starter solenoid and as I had to remove the intake manifold I got to wondering why there are eight ports on it (excluding the inlet of course) - the square ones into the head I'd expect but what do the round ones into the cam cover do?

[Phil-T4]

Swirl chambers

[Mike Noc]

Originally Posted by Sector-9

Just been out replacing contacts on my starter solenoid and as I had to remove the intake manifold.....

Hope you don't mind me asking, but why remove the intake manifold?

[Sector-9]

Didn't look like there'd be enough room to extract it with the manifold on, and it's a quick enough job to remove. Also found out the vacuum pipe to the EGR valve was disconnected on my 'spare' 75 which I'm currently using, and pretty much ruined on my laid-up 'daily' so I'll need to order a length to replace it with.

[Sector-9]

Originally Posted by Phil-T4

Swirl chambers

Ok, so basically each port feeds one inlet valve, and there are two inlet tract lengths per cylinder? I can understand the logic of this in a petrol engine where fuel is injected outside of the cylinder (turbulence helps to vapourise it), but how does it help in a diesel where fuel isn't injected until well after the inlet valve has closed?

[alanaslan]

Originally Posted by Sector-9

Ok, so basically each port feeds one inlet valve, and there are two inlet tract lengths per cylinder? I can understand the logic of this in a petrol engine where fuel is injected outside of the cylinder (turbulence helps to vapourise it), but how does it help in a diesel where fuel isn't injected until well after the inlet valve has closed?

A very good question. And a very well made point. I had always just assumed this was a common part to both compression ignition and spark ignition engines. But having looked up the parts listing it is not. Does anybody know the answer. You have now got two curious people out here.


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[Mike Noc]

Originally Posted by Sector-9

Ok, so basically each port feeds one inlet valve, and there are two inlet tract lengths per cylinder? I can understand the logic of this in a petrol engine where fuel is injected outside of the cylinder (turbulence helps to vapourise it), but how does it help in a diesel where fuel isn't injected until well after the inlet valve has closed?

Even after the inlet valves have closed and the piston comes up on the compression stroke, the air continues to swirl around inside the cylinder in a diesel engine.

This aids combustion and lowers emissions when the fuel is injected by helping to mix the fuel droplets with a larger volume of the available air in the cylinder.

[Sector-9]

Indeed it would swirl around but I'd have thought the rising piston would have a greater affect than the (now closed) inlet tracts? I briefly thought it was to help with scavenging exhaust gasses as there's often a period of overlap where both inlet and exhaust valves are open. Thinking deeper on that, it's not possible on engines where the valves and pistons interfere as all valves must be closed before TDC meaning there can be no overlap period. I suppose there could be more than one cam lobe per valve, allowing two valve openings in quick succession, or one inlet valve could be set to open just after the exhaust valves and then close before TDC, but I don't believe that's the case here.

One day they'll be able to use solenoid operated valves and do away with all the cams, cam driving paraphenalia and external EGR valves - but I suspect we'll be on electric motors before then.

[BigRuss]

One of the ports is the main charge port and the other is the swirl port, they are at different angles. The swirl port comes in more from the side and causes the injected fuel air mix to swirl round and so causes the fuel air mix to fill the combustion chamber. this makes for more efficient combustion

Russ

[Mike Noc]

Originally Posted by Sector-9

Indeed it would swirl around but I'd have thought the rising piston would have a greater affect than the (now closed) inlet tracts? .

In practice they both have an effect - even though the ports have been closed the air doesn't immediately stop swirling just because it is being compressed.