DIY_EFI Digest Wednesday, August 4 1999 Volume 04 : Number 449 In this issue: Re: DIY_EFI Digest V4 #448 More Questions on cranking TBI to Port Injection See the end of the digest for information on subscribing to the DIY_EFI or DIY_EFI-Digest mailing lists. ---------------------------------------------------------------------- Date: Wed, 4 Aug 1999 09:46:44 -0400 From: "Gary Derian" Subject: Re: DIY_EFI Digest V4 #448 This doesn't directly address the cold start issue but injecting onto a closed valve provides better atomization because the puddle of fuel is hit by hot exhaust just as the intake valve opens. 250 ft-lb of torque and only 111 hp at 6000 rpm? That doesn't seem right. Gary Derian > > Based on the last dynamometer run, we found we were at about 12ms at > 6000RPM and pulling 111 HP and our goal is 125HP; We've been able to > get over 250ft/lbs Torque. This, at a little less than 75% duty cycle. > As I understand the reason for 'the name of the game' is to provide a > smooth unloaded idle. Since we turn a prop idling at about 900RPM we > have a little bit of a load so a slightly larger injector might keep me > under 75%. (I have a hardware limitation on pulse widths longer than > 75% if I plan my injection at low rpm for the intake stroke). > > But, injection is supposed to get rid of the need for a lot of the > excess fuel because it's sprayed almost directly onto the intake valve. > Spraying onto a closed intake valve when the engine is cold or just > warming up doesn't seem logical for a low pollution fuel efficient > engine. ------------------------------ Date: Thu, 05 Aug 1999 03:31:24 +1000 From: Phil Lamovie Subject: More Questions on cranking Hi All, John wrote :- >I thought one of the injector's jobs was to atomize the fuel though. atomize = very small drops vapor = molecules Petrol drops burn like a candle. They have essentially two layers of combustion the ratio of which is controlled by chaos. As the drop surface vaporizes there are instants of both rich and lean combustion in very pretty patterns. The resultant gases are all bad news. Keep in mind the surface area/volume ratio. > (This is an remanufactured engine turning a propellor on a hovercraft or aircraft). > So we have to crack the throttle a little bit at startup. This has been the biggest obstacle to cold starting. If we open the throttle too far the MAP is > wrong compared to RPM and mixture is off. Don't even look at the Map sensor during cranking the signal from the sensor is useless as the engine is "cogging" and slowly to boot. The signal has both positive and negative excursions. For a high class test fit a vacuum gauge, and turn the engine over on the starter without fuel or spark. Don't borrow a gauge you can't afford to replace. Pick an appropriate amount say 7 ms and just squirt. If the engine fires it may go from 0 vacuum to 70 kPa and 600 rpm in 5 or 6 revolution. If this aircraft is flying above 5000 ft you would be well advised to reconnect the throttle body water supply. This will prevent freezing as the aluminum will out shrink the brass and lock it solid. This will make landings difficult and/or messy. Check your default for O2 sensor failure in closed loop and make sure it's not lean. Aircraft engines have a habit of operating under much higher average loads than cars. Thus a 70% load is not unusual. In most cars the average engine load is 10-15% This is not an appropriate load for 14.7:1 A/F in closed loop. If you have a broad band sensor then you will be able to target 12.5 - 13.0:1 A/F ratio. Under no circumstance should the engine boost at A/F of more than 13:1 it will simply melt after a few minutes. Imagine under what circumstance you would keep the throttle of a turbo car at 3/4 for 3 or 4 hours in closed loop. I know that auto makers are "dumb" but they have read the Bosch Blue Book. And the cost of the hoses was less important than a throttle stuck after a long run on the highway in freezing conditions. Remember that partial vacuum = cooling just as increased pressure = heating By the way how did you determine the injection amount vs map points for rpm vs load at 50, 100, 150 etc. rpm ? > Based on the last dynamometer run, we found we were at about 12 ms at > 6000 RPM and pulling 111 HP and our goal is 125 HP; We've been able to > get over 250 ft/lbs Torque. This, at a little less than 75% duty cycle. > As I understand the reason for 'the name of the game' is to provide a > smooth unloaded idle. Since we turn a prop idling at about 900 RPM we > have a little bit of a load so a slightly larger injector might keep me > under 75%. (I have a hardware limitation on pulse widths longer than >75% if I plan my injection at low rpm for the intake stroke). If you were firing the injector for 12 ms at 6000 rpm you were probably in another dimension. Lets see 6000 rpm = 100 rps or 1 revolution every 10 ms. I think you were missing injection cycles because you can't start the new event until you finish the old. The smaller the injector the smaller the spray and the better the atomization. etc. etc. as the drop is basically round the cube law applies in time vs droplet size. A drop with a diameter of 10% less has a mass of 50% less This stuff cost millions to learn with horrible cameras and engines with windows Take it as a given that the drops can't be too small. Why not use a single hall and a chopper with a differential width for the TDC indicator. Start a timer with the rising edge and stop it with the falling edge. The short one is TDC. > Spraying onto a closed intake valve when the engine is cold or just > warming up doesn't seem logical for a low pollution fuel efficient > engine. You use the words hot and cold like they mean something in an empirical sense. Express yourself in kelvin (K) and you hear a 20% change from "cold" to "Hot" Spraying into a sub atmospheric pressure gas stream , comprising one of the best insulators known to man that by the way has a terribly low specific heat seems like a waste of time unless the air is preheated. Some cars even use heating elements under the carby for this reason. Low pressure lowers boiling point but also gives less density and thus lowers the energy per specific volume of the air. Do the calculations on the heat that is required to vaporize the fuel and you will find that you need a generous donor. Heat is the issue and where to get it from is the problem. For that matter spray some on your hand. If you don't quantify your problem in SI units you won't come up with an SI answer. Question How many elephants fit in a phone booth ? Answer Some. If you leave out the units you won't pick the core issue The smoothest idle has the smallest cycle to cycle variation in combustion and that is hard to control if your target A/F is stoic. If you vary even a little say 15% leaner you will get a miss fire. This will decrease the vacuum level and cause the fragile chain to break. The rich tolerance for a "cold" engine is probably in the 100% range. The racer has to be separated from the idler. By all means PS When I say % I mean of course by mass not by A/F ratio. Regards Phil Lamovie injec@xxx.au cogito ergo zoom ------------------------------ Date: Tue, 03 Aug 1999 14:12:45 PDT From: "David Sagers" Subject: TBI to Port Injection I ran into a brick wall with my DIY EFI project this morning. I'm converting a '92 Suburban TBI to a '90 TPI MAP system. I have about all the parts including the TPI system, brackets, cables, etc... Called this morning to buy the wiring harness and the tech at Painless Wiring said that I was headed for a big problem. The speed sensor in the Sub runs the speedometer and something to do with the brakes. He said the problem is that the TPI ECM uses a different speed sensor than the TBI uses. And they are vastly different in the signals they produce, are not interchangeable, nor can you run a dual sensor system. I remember seeing in a Jegs catalogue that Edelbrock sells a multi-port system that uses the the TBI computer but requires Edelbrock's chip to make this work. I called Edelbrock and they do not sell this chip separately, only with the full EFI set up. The Edelbrock system was a possibility until; 1) the tech said that his system is not big enough to feed the 400 SBC I have ready to go in the truck. The injectors are only 19 lbs/hr; 2)and the tech said that the cam I using would require a custom chip anyway. I may have given him the wrong specs on the cam because I bought a mild towing cam for the Suburban; 3) I've already paid for all the TPI stuff and really like the long runners on the TPI, not sure I can really afford to shell out another $1000 for Edelbrock system. Some where in the box with the TPI stuff is a harness adapter that is suppose to convert the TBI injector harness to an eight injector harness for the TPI. But the big question. Does anyone know of a chip maker that makes a chip that will allow my TBI computer run a TPI system? _______________________________________________________________ Get Free Email and Do More On The Web. Visit http://www.msn.com ------------------------------ End of DIY_EFI Digest V4 #449 ***************************** 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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