Musings on fuel injection tech

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mikenixon
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Musings on fuel injection tech

#1

Post by mikenixon »

It's been a few years. I used to know more about this than I do now. :)

Intro
You know how a carburetor works. Air rushing through on its way into the engine passes through a calibrated space called the venturi. Low pressure is formed here and from it a tube is suspended into fuel that is vented to atmosphere. The difference between the fuel's atmospheric pressure and the venturi's less-than atmospheric pressure coaxes the fuel upward into the intake airstream. Thus we have "fuel discharge." From that point on, airflow changes generated by throttle operation bring proportional changes in venturi pressure, resulting in proportionally-different fuel discharges into the intake.

However, even a change in conditions as simple as cold starting requires outside, human intervention. Massive enrichening is necessary because the engine can't vaporize its fuel due to its low temperature. Much more fuel is needed because so much is wasted in trying to get a cold intake manifold to vaporize the fuel. This is challenging enough. Add an even more significant variation such as a change in altitude and the carburetor's capabilities are overwhelmed altogether.

The carburetor's weakness is that it operates solely on signals created by its own airflow to produce fuel discharge, and thus has no means of compensating for any change from baseline conditions. Its meager signals also result in mediocre atomization, the breaking of fuel into a fine mist, due to the fuel being under merely atmospheric pressure. Atomization is important because it is the first step in the combustion process.

Fuel injection adjusts to all those small yet important things the carburetor cannot sense. Plus, through external fuel pressurization, fuel injection breaks its fuel into much finer particles upon discharge than does the carburetor, resulting in improved combustion and power. Other benefits include reduced maintenance, better starting in cold conditions, and faster warmup. Fault override is also programmed into fuel injection systems, allowing them to keep operating even with one or more failed sensors.

The Parts
Modern fuel injection systems are controlled by an electronic control unit (ECU). A computer. Essentially a box full of three-dimensional electronic maps, estimates of the engine's fuel and ignition requirements, the ECU takes inputs from around the vehicle and matches them with with one of its maps. It gets these inputs from sensors. Sensors are the system's nerve endings. They convert the physical conditions of pressures, temperatures, throttle opening and gear position into electrical references. Most do this by accepting a small voltage from the ECU, modifying it according to conditions, then sending the altered voltage back to the ECU, which determines engine pressures and temperatures. The remaining sensors operate as thermistors, pressure-sensitive diaphragms, or variable resistors such as the throttle position sensor (TPS), which converts throttle angle into voltage. And there are still the simple on/off switch angle and gear position sensors, signal-generating sensors such as the rpm-counting crankshaft position, cam position and vehicle speed sensors, and the voltage-creating oxygen (O2) or exhaust sensor.

The four-stroke engine's crankshaft rotates twice per power cycle. The ECU has no way to tell the engine's compression stroke from its exhaust stroke. The camshaft position sensor adds half-speed information to the mix to divide the crankshaft's data by two, and thus provides the necessary distinction.

The fuel injection system's throttle body, which looks like the old-school carburetor, controls airflow exactly like a carburetor but contains no fuel.

Fuel injectors, merely electromagnetic fluid control valves, are mounted to the throttle body to deliver the fuel from the fuel tank. Pressurized fuel is constantly fed into them. The ECU operates them to dictate when and how much fuel enters the engine. A high-pressure fuel pump provides the push that gets the fuel through the fuel injector.

Operation
At startup, the engine temperature and air temperature sensors establish a starting mixture. As the crankshaft turns over, crankshaft and camshaft sensors communicate piston position, and the coolant temperature sensor tells the ECU to compensate for a cold engine. The ECU operates a fast idle mechanism for a temporary fast idle. When pulsed by the ECU, the fuel injectors open and the fuel escapes in a fog-like spray for a period of time that will deliver the mixture needs of the engine. The engine starts and idles. As the engine warms up, based on information from the coolant temperature sensor, the ECU gradually turns down the fast idle.

Once the vehicle is underway, rpm information from the crankshaft position sensor is added and the system runs on rpm and intake air pressure data. This low-speed or D-J mode is air density (D) based and has roots in Bosch Jetronic (J) technology. When rpm and/or loads increase, the system shifts to the Alpha-N mode, which is throttle position (Alpha) and engine rpm (N) driven. Early systems were called "open loop" and relied heavily on the ECU's maps. Current exhaust sensor based systems known as "closed loop" have ECUs containing fewer maps because the ECU adjusts the fuel mixture in real-time.

Diagnostics via laptop or handheld computers allow testing of injectors, evap valves, air injection solenoids, exhaust pipe butterflies, secondary throttles and more. Graphing, trends, real time sensor values, and individual part failure history are all accessible. Many manufacturers also allow reflashing of their fuel injection system ECUs for various purposes, including ECU updates.
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Re: Musings on fuel injection tech

#2

Post by rcmatt007 »

very informative.
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Re: Musings on fuel injection tech

#3

Post by bstuebing »

Thanks Mike. Your explanation really helps me understand my '85 LE (from a theoretical viewpoint). Is there anyone around who could be considered an "expert" on the '85 LE's and '86 SEi's ? My '85 has been dead nuts reliable but I live in fear of a fuel injection problem. I would have no clue where to start to figure out how to fix it.
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Re: Musings on fuel injection tech

#4

Post by mikenixon »

Hmm, Stu Oltman comes to mind. But he has retired. You might ask him who he knows is still servicing them. PM me for his phone number. He's here in the states.
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Re: Musings on fuel injection tech

#5

Post by Rednaxs60 »

bstuebing wrote:Thanks Mike. Your explanation really helps me understand my '85 LE (from a theoretical viewpoint). Is there anyone around who could be considered an "expert" on the '85 LE's and '86 SEi's ? My '85 has been dead nuts reliable but I live in fear of a fuel injection problem. I would have no clue where to start to figure out how to fix it.
Have an '85 LTD that is my retirement project and have been riding it since 2015. Have found the FI system to be robust and not in need of a lot of work. The only item I am concerned with is the ECU, but it was built to last as well. Have looked into after market replacements, of which there are really none, but the Megasquirt and Speeduino project are two options. I have been looking at the Speeduino project as a replacement ECU and have the bits necessary to start project. The biggest hurdle here is to keep the integration of the ECU, travel computer and dash.

The TPS could be an issue as these are no longer available, but there are several alternatives out there that have been documented on the various forums. The Ns sensor (crank) can be replaced with a set of PG sensors from an '85 Aspencade, bolt right in. The Gr/Gl sensor is not as readily available but there are sensors out there that have been used - the fellows on the CX500/650 turbo forum have found an alternative. The same fellows have found an alternative for the PB (MAP) sensors as well - a Suzuki product that is 99% same in performance (have had these installed on my bike for some three years - work well). Injectors are almost indestructible, a cleaning and flow test, put back in and ride. The spark igniters are not available, but used ones are out there. Honda built these extremely well for the time. The timing for these is quite interesting in how Honda has set it up, but that is another long explanation. Hope this helps.

Mike - apologize for the lengthy reply. Don't want to highjack your thread as I enjoy reading your words of wisdom.
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Re: Musings on fuel injection tech

#6

Post by mikenixon »

Not a hijack. Insightful input. :) Yup, folks make it happen, prove resourceful. I didn't know parts exchanges were taking place on the venerable FI model, but knowing the availability issues I can understand the motivation. Even as long ago as 2003 when I managed a Honda dealership, we had a lot of trouble trying to source 1200 FI parts.
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Re: Musings on fuel injection tech

#7

Post by Steven UK »

This is an old thread but interesting. I have recently fitted a fuel injection system to my 1963 Chevrolet truck. Trucks and motorcycles have little in common but the ECU has no bias or bigotry. The system I have used was the micro squirt that was originally designed for motorcycles. The ECU is programmable to use any input sensors and output transducers (injectors) My question is is anyone on here looking into such things. Carburetors and rebuild kits and the knowledge to repair them is drying up....

Also we need a recent post button on here... who do I need to speak to?

Regards Steve
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Re: Musings on fuel injection tech

#8

Post by Rednaxs60 »

For a recent posts button check under Quick Links at the top of the page.
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Re: Musings on fuel injection tech

#9

Post by mikenixon »

Steven UK wrote:This is an old thread but interesting. I have recently fitted a fuel injection system to my 1963 Chevrolet truck. Trucks and motorcycles have little in common but the ECU has no bias or bigotry. The system I have used was the micro squirt that was originally designed for motorcycles. The ECU is programmable to use any input sensors and output transducers (injectors) My question is is anyone on here looking into such things. Carburetors and rebuild kits and the knowledge to repair them is drying up....

Also we need a recent post button on here... who do I need to speak to?

Regards Steve
Cool stuff. Microsquirt and I think Megasquirt are well represented in grass roots motorcycle conversions. I think carbs are going to linger on motorcycles in the vintage segment for a long time, likely much longer than they have even on vintage cars.
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