Hi Daft question this one, am I right in thinking that one bank of injectors fires all 4 injectors at the same time on V8 engine(ie not sequentially).If this is the case why not do away with three of them and have one big one?
Al
In news:40cc757d$0$20512$ snipped-for-privacy@news-text.dial.pipex.com, alan blithered:
As in SPi? I think the answer is 1 injector bad, 8 injectors good, well better!
On or around Sun, 13 Jun 2004 16:40:45 +0100, "alan" enlightened us thusly:
they do on the hotwire system, not sure about the flapper one.
Assuming we are talking flapper or hotwire EFi here rather than the more advanced GEMS system....
There are 8 injectors in two banks of 4, they all "fire" at the same time with each of them, in theory, injecting the same amount of fuel. The firing point is random in as much as the ECU simply counts the pulses from the ignition system and fires the injectors every 8 pulses. It has no idea which cylinder is firing when it injects the fuel. The only thing the ECU has to figure out is how long to fire them for which, in turn, determines how much fuel is injected. It bases this on the readings from the air flow meter (more air, more fuel required), the throttle position and the engine temperature (and Lambda sensor if fitted).
The plenum chamber gives a common air input and some cross flow but each cylinder has it's own "trumpet", a tube that isolates the cylinders. When the injectors fire, the fuel charge for that cylinder is contained by the trumpet to prevent earlier firing cylinders sucking in more than their fair share of the fuel. The fuel is injected near to, and above, the inlet valve so as soon as the cylinder starts it's "suck" stroke the fuel charge is ready. Depending on the start point, there is always some loss of fuel to earlier cylinders in the cycle.
8 injectors, 1 per cylinder, is the only way to ensure an even fuel charge across all 8 cylinders. So, in answer to your question, 1 injector is not as efficient as 8. In addition there are practical issues... to use a single injector and allow high rpm you would need a much higher pressure fuel rail which, in turn, would require more complex fuel pumps and more robust fuel lines - all leading to more cost and less safety.Primitive LPG systems do actually use a single injector which is ONE of the reasons that LPG produces lower MPG figures and power than petrol on these engines. Later LPG systems have started using multi-point injection similar to the petrol system with improved MPG and power. The simple LPG systems though simply inject a constant stream of fuel into the airflow rather than injecting a measured amount of fuel per cycle. They act more like a carb than an injector.
HTH
cheers
Dave W.
On or around Mon, 14 Jun 2004 09:38:31 +0000 (UTC), Dave White enlightened us thusly:
I think you'll find the main reason for lower power/economy is the fact that the energy contained in a litre of LPG is less than that contained in a litre of petrol.
from what I've understood, a good single-point system with feedback (lambda) control will perform pretty much as well as multi-point gas injection. The biggest advantage with SGI is that it doesn't fill the inlet manifold with explosive mixture, which much reduces the potential for backfiring - backfires are bad news on flapper type systems (damage the AFM) and bad news also on modern engines with plastic manifolds, which can be broken.
half the reason for various more-complex petrol systems is to overcome the problems inherent in trying to mix a liquid fuel with air, although more accurate fuel metering is certainly an important aspect as well.
That's why I stressed the word "ONE" in "ONE of the reasons that LPG produces lower MPG figures and power than petrol on these engines", sorry if I didn't make that clear enough. There are a number of other reasons too but they aren't really relevant to the original question so I didn't go into them.
cheers
Dave W.
A single big injector doesn't give as good a mixture distribution to all the cylinders as multi point injection... very much like a carburettor. Another problem with big injectors is as the 'open' time gets very small, like at idle, they don't atomise the fuel very well so emissions suffer. Toby
All correct, except it counts 4 pulses then fires one bank of 4, then another 4 pulses later it fires the other bank. It doesn't fire all 8 at once.
The correct fiunction of the trumpet is to correct the airflow to the cylinders, preventing starvation of the "corner" 4 cylinders, due to the airflow pattern within the plenum chamber, the idea being to get an equal air mass flow to each cylinder. The effective length of the trumpet controls the torque characteristics of the engine.
Not so much loss, II'd have thought, more a case of poor atomisation if the fuel isn't injected into a moving airstream of sufficient velocity. There shouldn't be any flow up the ram pipe as there's no gas flow when the valve is closed - unless the valve seats are shot.
Badger. B.H.Engineering, Rover V8 engine builders. Landy 110V8 3.9 Auto, on gas. BMW 330dSE Sport Auto, on steroids!
In news:20040614150312395+ snipped-for-privacy@news.demon.co.uk, Dave White expelled:
Properly set up LPG (even single mixer) shouldn't give any power loss compared to petrol - the most important thing is to get the ignition timing and advance curve right for LPG as apposed to petrol. And if you run dedicated LPG then you can raise the compression ratio and get a useful power increase.
Correct, to a point. As long as you stick with a mixer type of system, you have a restriction at the mixer venturi, limiting the total mass air flow into the engine. Not by a great ammount, but probably slightly more than you can gain back by optomising advance curve and raising c.r. My 110 with a 3.9 V8 has a fully balanced and gas flowed engine, and there was a noticeable drop in performance above 3500 rpm (even when running on petrol) simply by fitting the mixer plate for the LPG. The strange thing is, this engine runs a modded dizzy with a LOT more mechanical advance straight off idle, an initial static advance of 12 degrees btdc, but the same total advance as std, and it still doesn't pink when running on petrol!!
Badger B.H.Engineering, rover V8 engine builders.
In news:camams$hft$ snipped-for-privacy@newsg1.svr.pol.co.uk, Badger expelled:
Use two mixers hung off a twin SU manifold and reduce the restriction or get a bigger mixer. I have had a lot to do with LPG conversions on Chev V8's and have seen dyno results that don't support your theory.
On or around Tue, 15 Jun 2004 09:10:45 +0100, "Badger" enlightened us thusly:
well, the latest toy is supposed to be timed at TDC, and when I went dual-fuel I timed it at 6 degrees advance - runs just the same on petrol with no obvious pinking. I might try winding it on a bit more.
On or around Tue, 15 Jun 2004 20:30:34 +1200, "EMB" enlightened us thusly:
the latest one I've done has a single 45mm mixer. It would in theory work with a single 50mm, but we weren't sure if the vapouriser I already had would cope. I reckon it probably would have, at that. However, it doesn;t seem to have hurt the performance unduly.
someone had a figure of 7-10mm² for mixer area, per BHP. The 3.5 injection in my thing is rated at a bit over 150 BHP ISTR, so that would give 43mm as a mixer size, using the 10mm² mentioned. I assume that's per BHP of the existing petrol system, I don't doubt that the 3.5 V8 could be tuned to give more than that, and would then need a larger mixer to suit.
Some very good information going round here methinks, but just another thought .Why didnt rover fit a cam sensor to the engine and then the injectors could open sequentially possibly giving a better control to the fueling (perhaps in the early days of the v8 efi around 1984/85 there were no processors capable of operating 8 independently@ 6000rpm :-)) Al
I don't think processing power had much to do with it, an engine running at 6000 rpm is hardly fast in electronic terms. We've had devices capable of processing data much faster than that in consumer electronics since the mid 70's. Admittedly they tended to be based on (relatively expensive) ASIC technology rather than CPU technology but possible none the less.
Asking questions like that is akin to asking why a 1920's car is so much slower than a 1970's car - they both use the same technology and you could have produced a 1970's engine in the 1920's technologically speaking but it was the 50 years of development and experimentation in between that made the 1970's car possible.
The latest Land Rover EFi units use cam sensors, crank sensors and knock sensors so not only can they use sequential injection they can change the mixture and ignition advance of cylinders individually and dynamically. Not just because of advances in technology but more down to building on the legacy and experience of the systems that went before.
Land Rover have generally been fairly near to the front of technology when it comes to EFi systems for years, especially when compared with other manufacturers. Next time you're in a breakers, take a look at how many manufacturers were still using carbs in the early 90's and how many of the mid 90's cars are using variations of the flapper EFi. You'll be surprised....
cheers
Dave W.
On or around Sat, 19 Jun 2004 23:45:59 +0000 (UTC), Dave White enlightened us thusly:
Flapper EFi isn't so bad though, if once set up right. If you're not going for a whole-hog eacH-cylinder-with-its-own-fuelling arrangement, I doubt that such as the bank-firing hotwire is a lot more efficient. I suspect the principal difference is that the hotwire can adjust to climate as well as mass airflow.
the 1986 V6 Ford I have here has Bosch K-jet - which is basic, flapper-controlled continuous injection but with electronic bells and whistles like an over-run cut-off, full-welly enrichment and so on. Once set correctly and driven tidliy, it can return quite good economy. 'course, drive it flat out and you're talking low teens. Previous owner reported 19 mpg on the computer (which, BTW, measures fuel-flow and relates it to road speed) at a steady 90 and 14 mpg at a steady 110. However, at more mundane speeds it used to do about 27 in ordinary farting around and on a steady run without silly velocity it's possible to get over 30. That's a moderatly large car with a 2.8 engine, and not bad.
by contrast, the 3.5 hotwire disco seems to return about 18 in averagely hard cross-country driving, about 16 pootling locally on small lanes. I daresay that it'd go over 20 with suitably careful driving, without actually driving purely for economy.
IMV the only real problem with the flapper system (as fitted to LR V8's) is the cold start system. Having a n injector who's only job is to fire fuel into the plenum chamber (almost) regardless of the engine's requirements at that time seems to be it's main weakness. It's amazing how many Range Rovers you see with the cold start injector unplugged.
The idle control left something to be desired too but the hotwire stepper motor isn't without it's problems either. As you say, the hotwire has the advantage of being able to measure oxygen content (air mass) rather than just air quantity so should meter the fuel more accurately. A flapper with lambda running closed loop should give just as good a result though at cruising speed/throttle at least.
Even better would be to attach a wide band lambda and ditch the air flow/ mass meter altogether :-)
My 3.9 hotwire Disco returns around 20 on motorway driving. Main problem is the aerodynamics and weight - my Range Rover used to return 22 mpg on motorway runs. When I moved the engine/transmission/axles/wheels under a
110 (which is actually lighter than the Range Rover) the fuel consumption dropped to just over 18 mpg on the same run. The only difference is the body shape.Have you ever thought of using a Megasquirt ?
I'm about to fit Megasquirt'n'EDIS to my 3.9 in place of the hotwire/ distributor. I spent a good part of yesterday machining a 36-1 crankshaft gear for the EDIS system, just need to make a harness up now.
cheers
Dave W.
Well, to be honest firing sequentialy or batch firing doesn't have as big an effect as you'd imagin! At maximum power the injectors will be open for something like 75% of the time comparied to the inlet valve beeing open about 25% of the time! Under these conditions it doesn't matter when the fuel injection period starts as fuel has to wate around behind the inlet valve for it to open anyway. Infact, injecting fuel with the inlet valve shut can be a benifit... the fuel vaporises on the back of the hot valve so giving a better air/fuel mix and this also cools the valve by a considrible amount. To inject all the fuel though an open inlet valve requires very large injectors and is only really posible up to the midrage of RPM. Toby
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