It’s a Gas

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Every internal combustion engine needs fuel, and a gasoline engine’s cylinders need a mixture of air and fuel vapor that is always in the range between 12.5:1 and 14:1. Normal spark ignition is able to fire mixtures between 18:1 and 10:1. Best power is given at 12.5:1. And chemically correct or “stoichiometric” combustion occurs at 14:1. Chemically correct means that fuel and air are proportioned such that the only products of combustion are water and carbon dioxide, with nothing left over.

Because gasoline evaporates poorly at lower temperatures, engines need a richer fuel mixture for cold starting, with “richer” referring to more fuel in proportion to air. If, for example, only one fifth of the fuel evaporates at a given temperature, then in order to simply start the engine the fuel mixture must be enriched by a factor of five. Before fuel injection, the device that accomplished this was called a “choke,” because it worked by restricting or “choking” airflow. This is why the exhaust of an engine with a carburetor smells strongly of raw fuel immediately after cold starting.

Flushing the Toilet

Carbureted motorcycles were designed with the fuel tank located up high enough that it could gravity-feed gas to the carburetor(s) located below. Each carburetor had a float bowl that worked like the float valve system in a toilet; it maintained a constant fuel height, and therefore fuel pressure. in the float bowl. As the engine used fuel from the bowl, “flushing the toilet,” a hollow brass or plastic-foam float would fall with the fuel level, opening the float valve to admit more fuel. As fuel rose, the float valve closed.

Carburetors were passive devices which used natural phenomena.The airflow rushing into the engine created pressure differences, pushing fuel into that airflow in the correct proportion. Because excessive vibration could froth the fuel in the bowl and cause variations in mixture strength, carburetors came to be mounted on flexible rubber sleeves.

Carburetors cannot automatically adjust to changes in ambient air pressure and temperature, so the mixture they supplied in summer was richer than that produced in winter. That problem was “solved” by jetting carbs rich enough to continue regular firing in the coldest expected riding weather. When the EPA tackled the problem of reducing unburned hydrocarbon emissions, motorcycle manufacturers had to lean out carburetion to the point that engines started poorly and offered weak snap throttle response; so began the era of “stutter-and-stall.” The first relief came from outfits like Dynojet, which supplied tiny washers which shimmed up the metering needles, thereby enriching mixture enough to avoid lean stumble. Reshaped needles also addressed the problem.

The Digital Revolution

The coming of digital fuel injection in the 1990s swept all that away. In place of a carburetor attached to each cylinder-head intake port there is now a fuel-injection throttle body. It contains an air passage for engine intake air controlled by a butterfly valve; a quarter-turn of its shaft covers the range from idle to full throttle. One or more solenoid-operated fuel-injection valves, or injectors, are provided to spray fuel into the air passage. Such valves are normally held closed by a tiny spring and are rapidly opened by an electromagnetic solenoid whose moving part is a tiny steel valve rod or “pintle.” Injectors are themselves “digital” in the sense that they are either open or closed; they do not meter fuel by partial opening. Because of this, the amount of fuel delivered by such a valve is entirely determined by just two things: 1) how long the valve is held open, and 2) the fuel pressure supplied to the injectors. There are no jets or metering needles as in a carburetor.

A compact computer called the ECU, or engine control unit, undertakes the job of determining when and how much fuel to inject. The computer needs to know crankshaft angular position at all times so fuel can be injected only during the intake stroke. A toothed wheel on the crankshaft, plus a stationary sensor, produce crankshaft position information continuously. The system must also discriminate between the intake and power strokes. This often requires another sensor on a camshaft, which rotates at one-half crankshaft speed.

To know how much fuel to inject, automobiles typically use an air meter to measure the mass airflow going into the engine. From that value the computer is able to calculate the injector on-time necessary to produce the desired mixture.

Although some early motorcycle fuel injection systems employed air meters (swinging-door, hot-wire, and so on), much more common are so-called mapped systems. An example of a new model is run on a dyno, and its fuel requirements are determined at all rpm and throttle angles. This information is stored as a 3D “map” or look-up table. (The three dimensions for the map are rpm, throttle angle, and injector on-time.) Shortly before an injector is to begin spraying, the ECU checks engine rpm “N” and rider throttle angle “alpha,” then looks up the corresponding injector on-time for that combination, which is stored in memory. An ECU “reflash” replaces the factory original stored data with another data set.

Because atmospheric density is always changing with temperature, barometric pressure, or altitude, other sensors adjust the injector on-time for conditions at that moment. Another variable is the specific gravity of the fuel itself, which can vary from about .680 to .760 (comparing it to water, whose density is defined as 1.0). If there is an oxygen sensor in the exhaust system, it detects any deviation from the desired mixture and the ECU corrects it.

In the carburetor days, more than one racer lost an engine by failing to measure his fuel’s specific gravity. Digital fuel injection is like having your own highly experienced professional race tuner constantly optimizing the operation of your engine.

If it seems impossible that so much computation can keep up with a 12,000-rpm engine, consider that the clock speed of some modern ECUs is 40 mHz; that is, the computer moves through 40 million switching states per second. Remember, too, that the speed of electronic signals is high and the dimensions of a computer chip are tiny. One revolution of a motorcycle crankshaft turning at 12,000 rpm takes 60/12,000 = .005 second, so during one turn of the crank the ECU can perform 200,000 clock cycles. Digital is fast.

Cold Starting Struggles

For cold starting, the mixture in a fuel-injected engine must be enriched, just as with a carbureted engine, to compensate for the fact that the heavier molecular species in gasoline do not evaporate at lower temperatures; gasoline is a mixture of as many as 200 different hydrocarbons, all within a specified boiling range. As the engine warms up and gradually is able to evaporate more and more of the fuel, the ECU leans out the mixture to keep the engine operating smoothly.

Because a higher idle speed is useful in keeping a cold engine from stalling, a throttle positioner raises idle rpm for starting, gradually reducing it as combustion in the warming engine becomes more stable. I sometimes sit in my little car after cold starting and watch the tach creep down from the initial high idle of 2000 rpm as warm-up progresses, thinking of the bad old days when motorists pulled out the choke knob to start and then gradually nudged it inward every few seconds until the engine would run smoothly with no choke at all.

The fuel reference pressure in a carburetor is the height of the fuel above the main jet in the float bowl. In a digital injection system the reference pressure is that delivered by a pressure-regulated electric fuel pump, usually located inside the fuel tank, keeping the lines supplying the fuel injectors full of fuel at the desired constant pressure, which is often around 60 psi.

In the early days of fuel injection it was not easy to make a single system cover the entire range of fuel flows from idle to peak power. Consequently, two alternative schemes might be used: One was to add a second smaller or “vernier” injector to the main injector downstream from the throttle butterfly. It would handle idle, and then hand off to the main injector at higher throttle. The other plan was to have two fuel reference pressures: a lower pressure for idle to just above, and then normal fuel pressure from there onward.

Evaporation Problems

Carburetors contain a free surface from which fuel can evaporate (the float bowls, which must be vented to atmospheric pressure), thus creating a chronic problem. If a bike is stored for a period, the fuel in the carbs may have time to evaporate, possibly leaving behind deposits that block the idle system. This requires removing, disassembling and washing out the carbs; not a difficult job, but still not something every motorcyclist wants to tackle.  Because fuel injection systems are pressurized, there is no free surface and no evaporation (other than inside the fuel tank), making it more likely that start-up after storage will be trouble-free.

When Dorna limited the amount of fuel a MotoGP bike could carry (a part of the organizational greenwashing we have all come to expect) Ducati experimented with cutting off all fuel flow during braking and at other times of zero throttle angle. They found that when fuel injection was resumed there was a period of irregular operation that was upsetting to corner exit. Fuel reaches engine cylinders by three routes—as evaporated fuel vapor, as entrained but as-yet-unevaporated droplets, and as “wall wash,” a slow-moving thin film of liquid fuel on intake duct interior surfaces. Switching off the injection allowed the wall wash to evaporate, so that when the rider began throttle-up that third transport mechanism (wall wash) was missing. That caused a temporary lean condition with possible misfiring.

A well-mapped fuel injection system gives such lovely response that test riders take notice and come close to writing poetry. The pace of modern business sometimes dictates that someone instead writes “Ship it!” before the last refinements are achieved. The result can occasionally be… Well, for want of a better word, prosaic.

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[Bender]

Fuel, air, spark bang,  that's all you need to know, move along...  

[Stu]

[Bender]

[Bender]

Pigs called Kevin, honest  

[MikeHorton]

Possibility of TMBF Kevin merchandise? 

[Guest]

15 hours ago, MikeHorton said:

Possibility of TMBF Kevin merchandise? 

This is such a good idea. 2022 tmbf Kevin calendar for a start.

[billy sugger]

On 18/12/2021 at 13:24, goat said:

This is such a good idea. 2022 tmbf Kevin calendar for a start.

How about a t shirt with "don't ask me ask Kevin!" on it