VIEWS: 7 PAGES: 8 POSTED ON: 7/26/2011
FUEL INJECTION (GENERAL & 4-STROKE) - page 1 of 8 All books and articles which are an idiot's guide to the functioning of carburettors start with a description of how a venturi (or "choke") works - by lowering the static pressure of the incoming airstream so as to suck up some fuel. This process is not efficient, it is known in thermodynamic terminology as "irreversible" - that is, you cannot get out of the process anything like the amount of energy which went in. My girlfriend is irreversible, for example. In effect, the engine is strangled purely to provide some power to drag the fuel in. Atomizing the fuel nicely (making a fine spray) so as to make it evaporate better takes more power, a more restrictive choke, which adds to the strangling effect. In addition, the chokes and spray bars play havoc with shock waves travelling up and down the inlet port, meaning that little advantage can be taken of their ramming effect. Worse, when a highly-tuned engine is "off the cam" and is suffering from blowback, the incoming air gets fuelled three times - once on the way in, again when spat out, and again when dragged back in. This accounts for the "stand-off" phenomenon - the cloud of petrol spray seen by carb bellmouths, totally messing up the settings. All of this doom and gloom begs the question, if chokes are so bad, why not pump the fuel in under high pressure, so as to get rid of them? The answer is that motorcycle design, even at the highest racing superstar level, is heavily influenced by stupid greasy oik stick-in-the-mud has-been old pillock luddite twats who much preferred life in the days of an old tobacco tin full of assorted jets, needles, and slides with different cut-aways. Kenny Roberts once said that carburettors are bound to supply the most suitable fuel mix for an engine as it then receives the fuel supply which its suction demands. Many journalists believed this and dutifully reported it, whereupon he must have gone back to the bar for a good laugh with the Yamaha engine men. When I was a development engineer working in the F1 car world, we also used to have competitions to see who could get a journalist to print the most ridiculous horseshit. My technique was to look slightly worried whilst describing some complete nonsense which I made up as I went along: it was amazing what the average hack would believe then, and we could read all about it in the next issue of Furry Dice Monthly. Another aspect of Mr Roberts' management technique, which he strangely forgot to mention in his superb book, is that when he finds something particularly good, he will tell everyone else that it is complete crap, thereby keeping the advantage to himself. Expect to see a fuel injected Yamaha GP bike soon. Page 2 of 8 The story which King Kenny put about is quite obviously false even now: modern carbs (2- and 4-stroke) have emulsion tubes, progression jets, power jets, air bleeds, and all sorts of other complex contrivances to fool the engine and provide anything but the mixture which it demands. By far the most blatant form of this meddling with the "natural" demands of the engine is the accelerator pump, which is actually a transient form of mechanical fuel injection. If you don't believe this, try opening the throttle with the engine switched off - there is no venturi effect, but the fuel still squirts out. Mechanical fuel injection was used in the car world on racing 4- stroke engines for decades with moderate success. Typically, the system would be not unlike an HGV diesel metering unit: a kind of pump, driven directly by the engine, which therefore delivered more fuel with increasing revs. The stroke of the pump would be controlled by a shuttle linked to the throttle system, normally with some sort of cam which allowed the part throttle fuelling to be controlled quite well. These systems never really translated into the bike world as they only worked well over a narrow rev range and the pickup could be rather abrupt. This is not a big deal on a machine weighing 500kg with a pair of 15 inch rear slicks, but is a lot more dicey on a two wheeler. The Norton-Cosworth twin was one of several bike engines to ditch a mechanical fuel injection system in favour of a pair of trusty Amal carbs. What a bag of shite that thing was. Conventional wisdom has it that poor management caused the death of the Great British Motorcycle Industry: in fact, that Norton engine proved that had the (highly esteemed) design engineers of the GBMI been Japanese, they would have been ritually beheaded by their shareholders. That even Cosworth couldn't sort that engine out goes to show just how bad it was in the first place. I worked in the Development Department at Cosworths for a while during the pimply stage of extreme youth. In those days I lived on beer and curry, my face looked like a pepperoni pizza and my ringpiece like a sliced blood orange. I was even more short-arsed then than I am now, and I used a Norton-Cosworth engine as a step to allow me to reach things on the top shelf in the stores - this is significant, as it is the only known case of one of these engine units being anything other than a total waste of space. The mechanical fuel injection system reached its height of development in the early eighties, the last days of the Cosworth DFV Formula 1 engine (Lucas mechanical injection) when the dog's dick road car to have was the Audi Quattro, which had a Bosch system, again totally mechanical. Page 3 of 8 It was then that fuel injection really came of age, with the increasingly popular use of microprocessor-based electronic control units (ECUs). These allowed for huge flexibility in controlling the fuel delivery, making the systems much more adaptable than the older mechanical ones. There are many concepts like this, which have been on the shelf as Good Ideas for years, they are just waiting for the technology to become available so that they can be made to work properly. An example is the Wankel engine, which has been around as an unreliable oily shitbox for ages, but has now been made viable with recent advances in tip seal materials and lubricants. The irony is that no sooner had it become workable than it was immediately banished to the outer darkness again by the environment people - a Wankel will never get through any half- reasonable emissions test, and will surely be banned for everyone apart from the military before very long. I wonder if a certain member of the Great British Motorcycle Industry has worked that one out yet? Probably not, they still do not know how to install a carburettor after eighty years of practice. The coming of the ECU allowed fuel flow to be controlled precisely by switching electrical shuttle valves (for that is all that a fuel injector is) for an easily controlled amount of time, thereby controlling the fuel flow. Most systems have a high- pressure fuel pump which keeps the supply side of the injector (known as the fuel rail) under pressure. A small regulator known as an FPRV (fuel pressure relief valve) dumps excess fuel from the rail back to the tank to keep the pressure constant, normally about 3 bar or 50psi, though there are a few 5 bar systems around. High pressure pumps are usually electrically powered, and made by a very rich kraut called Robert Bosch. Some racing engines have mechanically powered pumps, but these need to be spun over electrically to pressurize the fuel rail before the engine can be started, of course. Lucas make a superb one of these. As a safety feature, on road going vehicles the pump is located close to the engine to reduce the amount of high-pressure fuel in the system, and the supply is designed to be cut off by a relay in the event of an accident. Most cars have a low pressure or "lift" pump just to get the fuel out of the tank and up to the high-pressure pump. Page 4 of 8 The first really good electronic injection systems were made by Lucas. The ECU frequently controlled ignition timing as well, and was then known as an Engine Management System, or EMS. At the time, Lucas were world leaders in EMS technology, a reminder of the great days not so long ago when an engineer knew that if he couldn't find what he wanted in Birmingham, it would be a waste of time looking around the rest of the world. Some engineers at Lucas showed their warm appreciation of the progressive and sympathetic management style there by buggering off with all of the secrets to start their own companies, which flourished. To this day, outside the group of big multinationals who make their own systems (Lucas, Bosch, Marelli, Ford, Honda, etc) there is a plethora of small specialists who mainly concentrate on racing in a shifty sort of way because much of their stuff contravenes Lucas-held patents. Lucas seem not to mind too much as it all goes to help the sales of Lucas injectors, pumps, FPRVs, etc. The basic set-up of high-pressure fuel rail, injectors, and EMS (normally known as the black box or brain) has changed only in detail since the early Lucas systems. The main area of improvement has been in the brain itself, with the availability of more powerful and user friendly programming systems (software), and faster, more sophisticated chips (known loosely as "firmware", being neither hardware nor software. Really deep people, electronics engineers). The brain contains a series of instructions which governs fuel delivery and spark timing; these are normally worked out beforehand on a small computer and then "uploaded" to the ECU, where they are stored in a chip for use. The first chips, known as EPROMs (eraseable programmable read-only memory) were a bugger to use as the only way to alter the program was by pulling the chip out and putting a new one in. These could take anything up to half an hour to prepare (the process known as "burning in"). It was then normal to see F1 engineers with little foam-lined boxes full of chips, each with slightly different programs, which they could swap with whatever was in the car when required. My own experience of this system was that no matter how many chips you had, none of them was quite right, so that meant half an hour in the back of the transporter to burn another one in. The other approach (especially with Italian racing teams) was to stand around and argue passionately about which of the present chips represented the best compromise, but this normally took more than half an hour anyway. Page 5 of 8 The Great Leap Forward was then the EEPROM, or electrically eraseable PROM. This could be reprogrammed via a cable from a portable computer, meaning that a new chip program could be uploaded without any fiddly work in about 30 seconds. The cable (or comms link in computer jargon) also meant that an ECU could be controlled directly from the computer keyboard. This is especially useful in dyno testing, when all engine functions can be monitored, changed, and recorded on the computer, and the chip only programmed right at the end of the session. It is also possible to re-map car ECUs by driving up and down with a lap-top computer on your, er, lap. I have done this at 150mph in a racing Jaguar sports car whilst trying to cure a misfire, but have not tried it on a bike yet - I might be crazy, but I'm not stupid. The writing of software for an EMS and the subsequent programming of the ROM is a very highly skilled job demanding rare talent and experience: my guess is that only about one per cent of those people who think that they know how to do it really do, perhaps no more than 20 in the world at the highest racing level. What doesn't help is that there are only about five dynamometer systems in the world good enough to keep up with the latest EMS, so that tends to mean a lot of very expensive track tests. Money must then also be thrown at telemetry systems, because most test drivers and riders who are fast enough to be suitable seem only to get their IQ levels into double figures when going downhill with a following wind, and their feedback cannot be trusted. This is a pity, because modern EMS systems offer truly awesome control over engine functions, more than most engineers can use at present. As is often the case, adaptability also brings headaches, because with so many things to play with, so many things that can be altered at the touch of a button, setups are often way out and people just go around in ever decreasing circles until either they disappear up their own arseholes or they go back to carbs out of sheer frustration. Confucius said that a man with a clock knows the time, but a man with two is never sure. All modern software for programming ECUs is based on what is called the "map". A two-dimensional map is the easiest system, often used for ignitions. A sensor on the engine tells the ECU what the speed is, and the ECU will select the required spark timing from that point on its map. Thus, the programmer must enter a specific timing for every speed. To make things simpler, the program will require to know the timing required at (say) 250rpm intervals, and the software will join the dots up to decide what the timing should be between these intervals. Page 6 of 8 The next step is the 3-dimensional map, where the spark timing depends on another parameter as well as the rpm, normally the throttle angle, which the ECU can again find out by means of a sensor. Therefore, with a speed and throttle sensitive 3-d mapped system, the EMS is constantly checking these two inputs to find out what the spark advance should be by looking at the required point on the map - say 11,000rpm up and 30% open across. This is a very powerful tool; the advance for best power on full throttle at any and every particular speed can be found by dyno test and entered, the part-throttle timing can be advanced to provide good response and mid-corner pickup, the low-speed low- power setting can be retarded for easy starting, and a huge amount of advance wound on for high-speed closed throttle (ie overrun) conditions to stop popping and banging. Normally, modern EMS systems have two three-dimensional maps based on speed and throttle inputs, one map for ignition, the other for fuel flow. A typical 3-d fuel map will have high fuel flow for low-speed wide throttle (starting mixture), smallish flow on half throttle (to give a lean mixture for good economy and response) and a well rich mixture for best power on wide open throttle. Entering the fuel-flow values for every single point on a 3-d map takes ages, and so normally only a few points are checked on the dyno and the rest filled in by guesswork. This is why many racing engines run like ruptured ducks at low speed; the engineer simply couldn't be bothered to spend time testing an engine costing L75,000 with a service life of about 400 miles under conditions which the driver would never use. As maps for engines of the same type normally look pretty similar, there is usually a standard map which is fine-tuned to each engine in the important areas. "Important areas" on racing engines are wide-open throttle in the powerband; for a production engine "important areas" are idling and low-speed pickup, which are normally adjustable by screwdriver to keep the luddites happy. You would think that, with the ability to fix fuelling and spark advance at any possible combination of speed and throttle at the touch of a button, all engine management problems would be solved. The very first EMS systems did that, and were crap. There is so much more to engine functions than a pair of 3-d maps, and the surface has only been scratched. Again, the availability of advanced electronic systems only makes life for the engineer harder; it is no wonder that many just reach for the tin of jets. Page 7 of 8 Perhaps Karl Marx had this sort of thing in mind when he wrote in his Address to the Working Classes "....no improvement of machinery, no application of science to production, no contrivance of communication, ......will do away with the miseries of the industrial masses...". Perhaps not. The most obvious (but not the most important) problem with the map system is that there are many more factors to be considered in the fuelling and spark advance than just engine speed and throttle position: the temperature and pressure of the intake air are two factors which leap to mind. If the air is hotter and less dense, it will need less fuel to provide the correct mixture. This can be programmed in to be adjusted automatically, in the same way that a top racing team will put in smaller main jets and drop the needles a notch or two when going to Kyalami or expecting a heatwave. To save the complication of having a 5-d map to account for these extra inputs, a function is normally provided in the software to jack the entire fuel map up (for a richer mixture) or down. Of course, the provision of an inlet air pressure sensor means that the best possible mixture can be obtained in conjunction with a ram-air system, which increases inlet pressure with increasing vehicle speed. Other inputs which can be monitored and used to warp and stretch the maps (for just moving them is too easy, and therefore a bit passe' these days) are coolant temperature (an engine running hot cannot stand so much ignition advance), fuel temperature (if it's hot, it expands, so you need to keep the injector open longer to get the right amount in), detonation sensors, and exhaust gas oxygen sensors. If one of these sensors fails, or a connector in the loom comes apart, it can pull the maps all over the place, so the ECU is normally programmed to accept sensor readings over a certain range only: if a sensor goes out of the range then the ECU assumes it's knackered and uses a default setting instead. Of course, all of these sensors and systems have to be powered, and many modern EMS consume as much as 10 amps from the battery - which carburettors never did. Oops, starting to get into miserable old bugger mode. So now we have maps which will respond to any engine parameter that anyone can think of; the modern engine has more sensors on it than you can shake a stick at, surely that is adequate? No. Page 8 of 8 The trick which took a long time for people to latch on to (and which some still haven't caught up with) is that it is not just the value of any particular input at any time which counts, it is also the rate of change of that value. For example, the fuelling and ignition advance at, say, 30% throttle and 11,000rpm must be completely different if the throttle is being snapped open (for example blipping between down-changes), wound open gradually (coming out of a corner), or held steady (cresting a rise when there is no grip to transmit any more power). Anyone without L2m or so to spend on a state-of-the-art dyno will end up trying to set these transient parameters by the time- honoured system of going up the road, changing something a bit, then going up the road again. Other little tricks which the smart EMS can undertake are timed cold startup enrichment (like a choke), the provision of different ignition advance curves in each gear (less advance in the low gears makes response worse and therefore improves drive out of slow corners by making it easier to avoid high-siding and to keep the front down); different fuelling (richer mixture in high gears to stave off detonation); a "stutter" system (artificially low rev-limiter with the clutch pulled in to allow full throttle starts), etc. They can also control waste-gates on turbo systems, provide semi- automatic gearchanges (in the true sense, not just ignition cuts) and traction control through cutting the sparks to one or more cylinders or by "fly-by-wire" throttle control. The list is limited only by the imagination these days, and the lack of suitably intelligent engineers. It should be emphasized, however, that the single most important factor governing the pace of race engine development is....MONEY. Excellent state-of-the- art EMS black boxes can be had for well under L1,000, and the injectors, pump, loom, etc will bump the price for a full system up to about a gorilla and a half. The catch is that any EMS performs only as well as the man who programs it, and there are an awful lot of wankers around who think they know all about doing the job, but really haven't a clue and will blow your engine sky-high as soon as look at it. Mind you, some people don't need EMS to blow engines, they can manage that easily on carbs anyway. The select band of programmers who are "in the know" charge nearly a third as much as barristers for their services (gasp). A classic case of supply and demand, which puts Karl Marx's "from each according to his abilities, to each according to his needs" right out of the window. There is no doubt that racing, which exists to sort out one winner and lots of losers, is the ultimate capitalist system, and the technology involved is merely a reflection of that.
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