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My OEM n54 always targeted "low" (say ~700psi) DI rail pressures on idle and ramped up with load to ~2500psi.
From a tuning perspective with an aftermarket ECU I am having to consider calibration for the affects of this ramp, which is doable but adds another variable with error. What are the implications of running a constant target high enough for all situations? Let's say I target 2000psi across the rpm/load range. Since the pump is mechanical its not doing any extra work at idle, only the control valve is working at a different duty cycle but not by much.
Is there a good in cylinder reason to do with the injector spray behaviour for instance that needs to be considered, where a lower pressure actually helps with low rpm?
The sort of spread in target pressure is perfectly normal for a DI engine and is what you would want to target. The problem of running high pressure at idle or light throttle cruise is that you may struggle to achieve a stable AFR because the injector pulse width becomes so small. What is the reason you want to do this though? You shouldn't have trouble with the pressure tracking target even during fast transients as the system responds very quickly to changing pressure targets.
thanks, that's what I wasn't thinking about.
couple of things I observed, the minimum pump control duty the syvecs is set to 35%, which in my case wasn't low enough for closed loop to find the lower high pressure fuel targets at idle. I'm uncertain how critical the 35% minimum is with respect to a floor for operating the control valve properly. dropping the minimum duty floor to 30% seems to work.
the other thing I noticed was that the correction for injector duty versus rail pressure changes quite steeply at the very low pressures tapering off at the higher pressures. so getting those corrections correct for idle transition through the lower loads adds another tuning variable that's not a simple linear curve fit.
It was also on my mind that if the spray velocity and penetration was too high at idle speeds it might cause unnecessary piston wetting.
I just want to open a new thread about DI Fuel pressure.
The simple question is, what fuel targets should I use? Should I just look for longest injection time as possible (so as low pressure as possible)? What differences should I expect if I change the fuel pressure (given we are in a range, the injectors can control it with ease)
I think Iread somewhere in this forum, that it can be dangerous if your start of injection is too early. Or was it too late?
Unfortunately can't find this thread anymore. It's also possible Andre mentioned something in a webinar?
yeah there is a webinar on the topic covering timing - I recall comments about wasting fuel (too early with exhaust valve open) versus ignition timing limits (too late) as the window of opportunity. the webinar shows the pretty dramatic effect on torque from fuel timing - definitely worth watching again. I don't remember a specific warning about being too early.
Ok we have a couple of separate questions in here. Firstly @rac, if you can't meet the lower pressure targets (assuming they are sensible for a DI engine) then you could reduce the minimum duty cycle to allow this. It's quite possible that 35% is a touch high so I'd certainly suggest you experiment here. It's sensible to assume that you could just set the min and max duty at 0 and 100% respectively but the reality is that there will always be a non linear area of operation at both ends of the spectrum which is why we need some margin to achieve stable control.
@adrian, when you're choosing or manipulating fuel pressure in a DI engine it's all about managing the injector pulse width. At low fuel flow areas such as idle, too much fuel pressure will have the injector pulse width too short which can affect the injector's ability to consistently achieve the target AFR. Conversely at high load we need to consider the available window we can inject fuel into which might start close to TDC on the intake stroke and finish just before the ignition event. This is likely to be somewhere in the vicinity of 40% of the engine cycle time so at 7000 rpm this means we have something like 67 ms available to inject fuel. Obviously if you're needing a pulse width of 8-9 ms then you have a problem and more fuel pressure is often a solution.
In general there's little reason to change the stock DI fuel pressure targets at idle and cruise, however we will normally want to be as close to the maximum achievable pressure as we can get under high load, high rpm operation.
w.r.t to my feedback - yes I was right on the borderline with 35% min duty with the ECU needing 35% +/- 1% at idle, and lowering the minimum duty to 30% allowed proper control in achieving the lower pressure targets. while doing this AFR cycling around a mean value did tend to stabilise out a little at the lower injection pressures which demonstrates your follow on point regarding managing the minimum injector time.
Another option at idle is to retard the spark more (common on stock ECU idle controls) which increases the amount of air and fuel required for a stable idle. Normally from the factory these engines don't idle at MBT spark. They retard from MBT as a baseline, and then add spark when accessory load comes on. Since the engine isn't as efficient with retarded spark, more air and more fuel is needed, and you don't reach the lower pulsewidth limit.
Thank you for the answers. I tried today to vary the pressure at low loads and RPM. It doesn't seem to have any effect on power output. According to a scientific report, the emission particles are less and smaller as higher the injection pressure is. On the other hand, it needs a bit more power to run the High-pressure pump. So think at the end of the day, we don't have to bother too much about Fuel pressure targets in the aftermarket, as long the injector timing is in range, which allows good control of the injector.
...Regarding the other question. I can think of that injection in the early exhaust cycle is a bad idea and can probably lead to a second combustion, if there is enough oxygen left.
At part load it doesn't need to be very precise because we're not trying to squeeze out every last drop of fuel economy and emissions reduction. Generally at WOT or at least in boost on turbo engines you want to go to max rail pressure. A general range for injection timing on a standard single injection per stroke type of fuel system is between about 220 and 340 degrees BTDC, with 280-300 being a sort of general rule of thumb that works for a lot of engines.