Logic of injector sizing?

[John Howe]

Can anyone help with an equation or the logic behind sizing injectors. I assume with a fuel regulator pressure of say 2.5 or 3 bar there must be a calculation relating to cylinder capacity, trageted bhp and also the necessary fuel pump rating.

 

JH

 

 

[EFA]

Flow rate in CC/min divided by 4 is the bhp per injector max figure at 80% duty cycle.

 

Pump must be able to provide the sum of the flow rates at the given pressure.

 

Most injectors are quoted at 3 bar, but most will operate at much higher pressures.

 

Fat Arn

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[yankeedoodoo]

There is a formula on the twminduction.com site for sizing injectors. Don't know if it's dung or just what you need (hopefully you don't need both).

 

 

[yankeedoodoo]

An additional question: Would choosing an oversize injector, such as one that operates @ only 60% @ WOT, cause any specific problems?

 

 

[Bob Corb]

Injectors have a minimum pulse width, below about 2ms the characteristics of the injector go out of the window. Therefore on light throttle at low to medium engine speeds the fuel delivery will become erratic and accurate control of AFR is difficult. This will lead to some driveability and emission hiccups. A decent calibration engineer should be along soon to explain this better.

 

BC

[AMMO]

I have on my desk an injector from a GSX 1300 R Hyabusa. The same injector is aslo fitted to a GSXR 1000, the GSXR 750 and GSXR 600.

 

The Pico 330 cc injector has been fitted to the GSXR 750 World Superbike, 170 bhp at 15,000 rpm, 300 bhp Touring cars at 8,500 rpm, Peter Carmichael's K-series, 250 bhp engine and the MG Le Mans car fitted with the 2 litre turbo which produces 520 bhp. The latter runs about 10 bar fuel pressure.

 

In the real world when an engine goes on the dyno if the injector doesn't deliver enough fuel when running flat out, the fuel pressure is increased. If this doesn't do the trick and the fuel pressure cannot be safely increased any further, then the next step is to go for a bigger injector.

 

Selection is further complicted if you are running sequential rather than non-sequential injection. In sequential system the fuel is delivered in one shot every 720 degrees as opposed to roughly the same amount of fuel in two shorter pulses every 360 degrees. Usually the pulses are 51 or 52% of the sequential pulse as non-sequential isn't as efficient. I know this from the mapping of the GSXR 750 to run in a non-sequential default mode in case of a cam sensor failure.

 

As rpm goes up the available fuel duration pulse is reduced so rpm is a major consideration.

 

My point is that the selection is complicated by different factors.

 

The easy solution is to find someone who is successfully running the same engine as yours and find out what they are using and at what pressure. It's sort of cheating, I know, but at roughly £60 each, it's better than buying the wrong injectors.

 

 

 

AMMO

[oilyhands]

The only real problems with larger injectors are

i)that the granularity of the adjustment is larger so it may not be possible when fuelling requirements are low to exactly meter the fuel required, one fuelling value may be too lean, the next up too rich etc.

ii)the spray pattern may produce a droplet size that is not quite so easy to homogenise with the airstream, especially at low duty cycles where air shear is low.

 

When you consider that the stock K series injector is capable of supplying enough fuel for 220BHP, then at its maximum duty cycle in a stock 1800K it is well below 60%.. and the engine exhibits no real vices associated with fuelling.

 

Oily

[AMMO]

Oily

 

So your conclusion is that there is no problem with running an injector at 60% duty cycle and that it still runs OK at low rpm. I agree with this.

 

Out of curiosity when you use the stock K-series injectors with 220 bhp is the fuel pressure increased? What is the stock fuel pressure on a K-series anyway?

 

An interesting feature of the Pico injector is that it has four jets rather than one. It think it has a diaphragm type system rather than a pintle. This probably helps fuel droplet size at low rpm.

 

AMMO

[oilyhands]

Ths stock fuel rail pressure is around 3 bar, to get to 220 it has to be increased to around 3.5-4.0 bar. Many of the larger rated Bosch injectors are multiple nozzle, this certainly helps with fuel atomisation.

 

oily

[Bob Corb]

You should be aware that ECD3 K injectors wont be up to 220BHP. The injectors were revised as part of the change to ECD3. Basically they are now lower flow which improves the control of fuelling at low load, hence reduced emmisions.

 

BC

[oilyhands]

Thanks.. I already know, but I wouldnt recommend use of a particular injector categorically without first confirming its identity using the Bosch part number.

 

Oily

[Peter Carmichael]

There was a second part to your question. Fuel pumps generate both pressure and flow rate. The fuel pressure regulator *spills* fuel back into the tank so that:

 

fuel needed by the injectors + fuel spilt = the fuel rate at which the desired pressure is achieved.

 

A fuel pressure regulator has to spill almost all the fuel pumped at idle and much less at full power.

 

To get 250bhp from grouped injection with 330 cc/min Pico injectors required setting the fuel pressure up to 4bar (57psi). This required the uprated JPE pump from Caterham. Duty cycle gets up into the low 90s at peak power.