DIY_EFI Digest Wednesday, October 6 1999 Volume 04 : Number 564 In this issue: low pressure cfi Re: DIY_EFI Digest V4 #562 IAC valve and motor body needed FI for Mini See the end of the digest for information on subscribing to the DIY_EFI or DIY_EFI-Digest mailing lists. ---------------------------------------------------------------------- Date: Wed, 6 Oct 1999 09:07:10 -0400 From: "Clare Snyder" Subject: low pressure cfi I'm trying to put together a minimalist efi for a Soob aircraft conversion. Looking at GM single unit TBI from a 2000 cc 1988 OHC Pontiac. Is this a 10psi injector? What flow rate? I'm thinking a 555 triggered by ignition pulse with the base pulse set for idle or no load conditions. A MAP sensor output fed to the control pin of the 555 will increase the pulse width with engine load. I have several Toyota ZZ map sensors to play with - they run on 5 to 8 volts and output a proportional voltage. On a breadboard I mocked it up and it looks like the scaling should be close. I plan on using a Cherry injector driver - peak and hold. Any insights? For installation in the plane I would use a small header tank with a float (from a holley) fed by low pressure pump from main tanks, with EFI pump in the header tank, and fuel return to the header tank. High pressure (oh wow, 10 PSI!!) would be confined to the short line from the pump to the throttle body. ------------------------------ Date: Wed, 6 Oct 1999 09:27:25 EDT From: A70Duster@xxx.com Subject: Re: DIY_EFI Digest V4 #562 Earlier Haltech units used engine RPM, MAP, TPS, air inlet and engine temperature sensors and ran fine! That is all you should need to get going. Mike ------------------------------ Date: Wed, 6 Oct 1999 09:47:21 -0700 From: "Kavanagh, Jason" Subject: IAC valve and motor body needed I'm searching for an idle air control valve and motor body. The valve is the GM type used in the '88 Celebrity: o-ring style; bolt-on; in-line 4 pin connector. It is Borg-Warner model #21755. The motor body is the one supplied by Electromotive, part number 81112. Does anyone have these laying around and wants to part with them? Jay not currently subscribed, please respond directly ------------------------------ Date: Wed, 6 Oct 1999 11:03:05 -0700 From: "John Dammeyer" Subject: FI for Mini Hi All, My two cents here after finishing off a FI system for a Honda. I did this incrementally with a minimum of sensors to start with. What you are really interested in is calculating at any RPM the amount of air that will flow into the cylinders at any RPM. To determine the flow and therefore the air mass you need to know the pressure outside the throttle plate and the pressure on the other side of the throttle plate inside the manifold (MAP). If you assume standard pressure and temperature you end up with only needing MAP. An O2 sensor helps here for setting up a volumetric efficiency table. Calculate the amount of fuel the engine will need for 14.7:1 air fuel ratio given cylinder size and injector size. This is your starting point and is usually a number in milliseconds that you convert to timer ticks so your micro can program a counter to trip on and off the injector. This value assumes 100% volumetric efficiency. i.e. when the intake valve closes and the piston is a BDC the airpressure inside the cylinder is atomospheric. (almost never happens except at cranking with throttle wide open and cranking is a special case) Next determine the ratio of MAP/MAPWOT. Assume MAPWOT is the same as atmospheric pressure (emperically it's usually 1/2" Hg less than Atmospheric (30.50"Hg)). You can also determine MAPWOT when processor first starts up before engine cranking begins; MAP == MAPWOT. This ratio, always less than 1, is a quick way of calculating the mass of the air being pushed into the intake manifold by the external air pressure. Remember, the engine doesn't really suck in the air. It creates a low pressure zone and the higher external air pressure pushes the air in to fill the low pressure zone. Just like our weather systems. The difference in air pressure determines how fast the air moves and air always flows from a high pressure zone to a low pressure zone. (Can anyone spell hurricane?). Now, because air has mass, it doesn't start moving right away and doesn't stop immediately either. At idle RPM the piston is moving slowly so the change in MAP is also slow. This means with a closed throttle and low MAP (10"Hg) not a lot of air gets into the cylinder. The air movement has a chance to slow down and stop moving before the next cylinder intake valve opens. At high RPM the piston creates this low pressure area much faster. Although the throttle may be totally open, and there is a small difference in MAP verses MAPWOT, the air is moving quickly and doesn't have a chance to stop moving before the next cylinder needs air so each cylinder is filled with more air molecules. Therefore the amount of air that gets into the cylinder at any RPM is called Volumetric Efficiency and is somewhat independent of Throttle position. You can use 200RPM steps and a single dimensional table that needs to be calculated for VE. Values range from about 0.3 at idle to 0.8 at WOT for street engines and can exceed 1.0 for race engines with wild CAMs. So the formula is PulseWidth = FuelAirRatio * (Map/MAPWOT) * VE[RPM]; This works but doesn't handle acceleration very well because the opening/closing of the throttle can lean/richen the mixture before the math catches up. So to make the engine drivable, it's a good idea to use a Throttle Position Sensor (TPS) to add an enrichment parameter to the formula based on both absolute throttle position and change in the TPS (dTPS). You'll also find you need to use a different FuelAirRatio for idle verses cranking verses running verses WOT. A couple of extra points here. Air temperature changes the density of the air so an air temperature sensor will help set the mixture more precisely. An external air pressure sensor is also important because the vehicle may change altitude without the engine stopping and restarting. i.e. When you start, you know atmospheric pressure because the MAP sensor reads static air pressure before the engine starts turning. But once the engine is running you may change altitude by 5000' during a 1 hour drive. At that point the Atmospheric Pressure is considerably less than it was when the engine was started and so there is less external pressure available to push the air into the intake manifold and so for a given MAP value, the mixture will be too rich. This is why pilots always have to lean out the carburator mixture as the aircraft climbs in order to avoid fouling the spark plugs. It's really that simple but implimentation and tuning etc. is still a lot of work. Don't take it all lightly. The O2 sensor will give you a guide as to where you are but the stock auto ones are notoriously inacurate on either side of 14.7:1. A second method of determining optimum mixture at high RPM is to use Exhaust Gas Temperature (EGT). A K type thermocouple inserted into the exhaust manifold as close as possible to the valves results in about 1400F at optimum power. A temperature of 1600F suggests a too lean mixture and could potentially damage the engine and a 1200F temperature is too rich. Whew! There's four months work in explained in a few paragraphs. Regards, John >>Subject: How completcated does Efi HAVE to be? >> >>I am into minis, something a lot of americans probably won't have even >>heard of never mind seen! Up until recently minis had single point throttle >>body injection. You can pick up the inlet manifold, throttle body with the >>following sensors, The system is a MAP system. I was just wondering how few >>sensors I need to make it work. Make my own ECU and keep it simple. >>Calibration would be done on a dyno using a lap top. No need for a Heated >>o2 sensor, don't need it to learn really. > >There is a EFI Control System called the Alpha-N which calculates air flow by >measuring only engine rpm (via coil) and throttle position via a Throttle >Position sensor. Pros are simple installation and compatible with radically- >cammed engines. Cons are no closed-looped operation, inferior part-throttle >cruise metering and entire fuel map must be reprogrammed. A MAP can be >optionally >used but mainly as a input for barometric pressure for different altitudes. >This system is used for racing or heavily-modified street engines with big >cams >and low vacuum. > >Bob, Burlington, Ontario >'97 FR CC Sport, 5.2L, 3.55 SG, auto., 15.234 @xxx.26 > [snip] >------------------------------ > >I vaguely think that you only need a throttle position sensor and a map sensor >to do a reasonable job of EFI. However, you WILL need the crankshaft position >sensor, which means you need the flywheel, transfer case and the actual >sensor. >- -- >Tom Parker - parkert@xxx.nz > - http://www.geocities.com/MotorCity/Track/8381/ > ------------------------------ End of DIY_EFI Digest V4 #564 ***************************** To subscribe to DIY_EFI-Digest, send the command: subscribe diy_efi-digest in the body of a message to "Majordomo@xxx. 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