Injector pipe length

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bobroberts
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Injector pipe length

#1 Post by bobroberts » Thu Sep 24, 2020 11:57 am

Newbie question - I have a WJ V8 grand cherokee with an e- g@s system fitted years ago. It has injector pipe lengths from matrix to manifold of differing lengths. Should these be of equal length as I have seen on some YouTube install vids ? Many thanks.

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Re: Injector pipe length

#2 Post by Fox vehicles » Thu Sep 24, 2020 4:53 pm

Hi, it is often advised that the pipes should be of equal lengths. In reality it makes little difference to the running of the engine, the explanation for this is that when the system switches the pipes are filled with gas, the gas that is pushed from the injector, pushes a equal amount out of the end of the pipe (we have to remember this is all happening very quickly) a bit like a row of buses, with the back bus pushing the front bus. So generally a neat pipe run is more important that equal pipe lengths. I once did an experiment on my Range Rover with a 1meter length of injector hose, just on one injector, and sensible pipes on the other 7. Just for a bit of fun, as ya do, :D On change over you could tell a small blip on that cylinder, but once running there was no obvious difference. I would say sensible pipe lengths and manifold spud position are far more important than equal pipe length.
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bobroberts
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Re: Injector pipe length

#3 Post by bobroberts » Thu Sep 24, 2020 6:47 pm

Thanks Fox Vehicles. Apart from the unequal pipe lengths everything else appears as you describe. Cheers for the info.

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Re: Injector pipe length

#4 Post by LPGC » Fri Sep 25, 2020 1:53 pm

My advice would be much the same as above advice - it's usually better to aim for short pipe lengths than equal pipe lengths.

I think the row of busses analogy is correct up to a point, mostly correct for constant driving conditions and on an engine that doesn't feature anything like an Atkinson cycle... But still if the example of extending pipe lengths was taken to a further extreme there would be increased negative effects on driveability during changing throttle positions (and on some engines a lot more than others).

The pipe volume can be considered part of the manifold/plenum volume, pipes will be at much the same pressure as the manifold except for the pulses of higher pressure gas from the injectors. If there's gas flowing through pipes in these pulses from injectors with general pipe pressure being much the same as manifold pressure... but then manifold pressure changes... it will effect the flow of gas through the pipes - e.g. if during a low throttle cruise the driver increases throttle, manifold pressure will increase, pressure in the pipes will therefore increase by way of air from the manifold flowing backward up the pipes from the manifold towards the injectors, during this backflow the amount of gas coming out of the pipe into the manifold will be decreased (because the front busses are trying to be pushed backwards by the incoming air from the manifold), the effect momentarily leans tip-in mixture (not ideal and this can effect driveability during tip-in). During lift-off (deceleration) if there's a volume of higher pressure gas in the pipe between injectors and manifold this gas will be drawn out of the pipe into the manifold as manifold pressure decreases, the effect momentarily enriches lift-off mixture (not ideal but this is unlikely to be of much consequence).

Atkinson cycle and some VVT engines can have drive-ability effected more by long pipe lengths than older engines because some of the intake fuel is pushed back out of the cylinder through the intake valve, so during changing conditions such as tip-in (increasing throttle / manifold pressure) it is more important that any sudden increase in fuelling that the cylinder needs to have correct mixture doesn't reach the intake valve much later than it does when running on petrol. If a pulse of gas reaches the intake valve a bit later than a squirt of petrol from a petrol injector would it can not only be effected by initial decreased amount of gas coming out of the point of gas entry into the manifold but can also result in more of that gas that does make it into the cylinder being pushed back out of the cylinder again before the intake valve closes, a double whammy in terms of negative effect on fuelling and drive-ability.

On some installs with long pipe lengths you can see the effect in terms of bit of hesitation during changeover from petrol to LPG. If the engine has been sat idling for a while on petrol all the gas in the pipes between the injectors and manifold has had time to mingle (natural mixing of gasses that are at same pressure) with air in the manifold, so when you switch to LPG the pipes at first contain no gas (no busses) and hesitation occurs until the first busses start coming out of the end of the pipe and reaching the cylinders. If the same engine has been run on LPG and without turning the engine off is switched briefly to petrol then quite soon switched back to LPG there's no hesitation because most of gas in the pipes hasn't had time to naturally mix with air in the manifold. Some ECU's allow a bit of compensation for pipe length between injectors and manifold. Often termed 'fuel overlap', during switching from petrol to LPG it will continue pulsing petrol injectors (up to a settable number of milliseconds for a settable number of injection pulses) while pulsing gas injectors at the same time. The idea is that it gives pipes between injectors and manifold time to bleed up (get full of gas / busses) so that fuelling isn't interrupted when switching from petrol to LPG with (usually) quite long pipes but such system cannot help with other driveability issues after the engine has switched to gas and settings have to be made carefully to compromise between unsmooth changeover due to too lean mixture when the pipe between injectors and manifold has been purged of gas by running on petrol for a while (or changing manifold pressure, or engine has been switched off) and too rich mixture when the system has been running on gas, has only briefly been switched to petrol and then been switched back to gas again (pipe still full of gas so if petrol injectors pulse at the same time as gas injectors the mixture is too rich). It also has to be set carefully to work as well when the engine is under load as it does when the engine is at idle, and when the engine is relatively cold (longer injection pulses at low loads than when the engine is warm) as when the engine is warm. Still, fuel overlap works best with short to medium pipe lengths and the longer pipe lengths are the more difficult it can be to find the best compromise in settings.
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