Regan Brown
Fuel injectors are used to deliver fuel to an engine. Injectors have a point in their operation at low pulse widths that will start to make the fuel flow coming out of the injector nonlinear. Nonlinear fuel delivery means that the injector will no longer deliver the exact amount of fuel that is commanded to flow. This can be catastrophic for engine survival. The quality of the injector internals and spray pattern both play a role in the linear response of the injector.
| Characteristics | Values |
|---|---|
| Injector size | Injectors of the same size may not perform the same |
| Injector internals | The quality of the injector internals affects the linear response of the injector |
| Spray pattern | The spray pattern affects the linear response of the injector |
| Low pulse widths | Injectors will start to make the fuel flow coming out of the injector nonlinear at low pulse widths |
| Nonlinear fuel delivery | Nonlinear fuel delivery means that the injector will not deliver the expected amount of fuel |
| "On the cliff" | Injectors will have linear fuel delivery with pulse width until a certain point ("the cliff") |
| Duty cycle | Injectors do not typically operate the same at the extreme ends of the output cycle |
| Non-linear fashion | Injectors operate in a non-linear fashion at low output and the top end |
| Non-linearity | EFI systems have tables to correct non-linearity, but it is best to avoid operating the injector in these ranges |
| Injector variables | Injector variables can be inherent and affect performance |
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What You'll Learn
Injector size
Fuel injectors are responsible for delivering fuel into the engine's combustion chambers. If the injectors are too small, the engine won't get enough fuel, which can lead to performance issues. If they are too large, it can result in too much fuel being delivered, causing inefficiency and potential damage.
The quality of the injector internals and spray pattern both play a role in the linear response of the injector. Injectors do not typically operate the same at the extreme ends of the output cycle as they do in the middle of the flow curve. At low output (typically below 1.0 to 1.4 milliseconds of pulse width) and the top end (above 85 to 90 per cent), injectors operate in a non-linear fashion. This means that the injector can no longer be trusted to deliver the exact amount of fuel that is commanded to flow.
To calculate the necessary fuel injector size, a formula is used that factors in the estimated maximum horsepower, brake specific fuel consumption (BSFC), and the number of cylinders. The fuel injector size in pounds per hour (lbs/hr) is derived by multiplying the estimated maximum horsepower by the BSFC, followed by dividing the product by the result of multiplying the number of cylinders by 0.8. This can then be converted to cc/min by multiplying the lbs/hr by 10.5.
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Injector internals
In general, the larger the fuel injector, the lower you will have to reduce the pulse width to achieve the same AFR as a smaller injector. At low output (typically below 1.0 to 1.4 milliseconds of pulse width depending upon the injector) and the top end (above 85 to 90 per cent), injectors operate in a non-linear fashion. This means that the injector can no longer be trusted to deliver exactly the fuel that is commanded to flow. If less fuel is delivered, this can be catastrophic for engine survival at WOT.
To allow for inherent injector variables, it is common practice to reduce the injector’s duty cycle. An injector that is held open continuously would be rated at 100 per cent duty cycle. At a 50 per cent duty cycle, the injector is flowing fuel only half the time. For a normally aspirated mild street engine, Holley recommends sizing fuel injectors where the maximum duty cycle is limited to a maximum of 70 per cent.
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Spray pattern
Fuel injectors are not always linear. Every injector has a point in its operation at low pulse widths that will start to make the fuel flow coming out of the injector nonlinear. Nonlinear fuel delivery means that if we expect the injector to deliver 40 cc/min at 1.0 ms (millisecond) pulse width, it should theoretically deliver something like 35 cc/min at 0.8 ms. However, this is almost always untrue. Every injector will have a point where it's "on the cliff", meaning there will be linear fuel delivery with pulse width until a certain point. This is usually in the lower and upper 5 to 10 per cent of their flow ranges.
The spray pattern of a fuel injector is an important detail that affects the linear response of the injector. The quality of the injector internals and spray pattern both play a role in the linear response of the injector. The spray pattern is the way in which the fuel is sprayed into the engine. It is important that the spray pattern is even and consistent so that the fuel is distributed evenly throughout the engine.
There are a few different types of spray patterns that can be used in fuel injectors. One common type is the cone spray pattern. This type of spray pattern creates a cone-shaped spray of fuel that is directed into the engine. Another type of spray pattern is the fan spray pattern. This type of spray pattern creates a fan-shaped spray of fuel that is also directed into the engine. There are also other, more specialised spray patterns that can be used in certain applications.
The choice of spray pattern depends on a number of factors, including the type of engine, the fuel being used, and the specific application. It is important to choose the right spray pattern for the specific application, as this can affect the performance and efficiency of the engine.
There are a few things to consider when choosing a spray pattern for a fuel injector. One important consideration is the size of the droplets. The size of the droplets affects the way in which the fuel is atomised and distributed throughout the engine. Smaller droplets tend to be more easily atomised and distributed, while larger droplets may be more difficult to atomise but can provide a more concentrated stream of fuel. Another consideration is the angle of the spray. The angle of the spray affects the way in which the fuel is directed into the engine. A wider angle can provide more even distribution, while a narrower angle can provide a more concentrated stream of fuel.
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Non-linear fuel delivery
At low output (typically below 1.0 to 1.4 ms of pulse width) and the top end (above 85 to 90 per cent), injectors operate in a non-linear fashion. This means that the injector can no longer be trusted to deliver the exact amount of fuel that is commanded to flow. If less fuel is delivered than expected, this can be catastrophic for engine survival.
Non-linear behaviour can occur at light throttle driving, depending on the injector characteristics. In general, the larger the fuel injector, the lower the pulse width must be to achieve the same air-fuel ratio as a smaller injector.
Although most EFI systems have tables to correct non-linearity, it is best to avoid operating the injector in these ranges. To allow for inherent injector variables, it is common practice to reduce the injector's duty cycle.
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Injector duty cycle
Injectors do not typically operate the same at the extreme ends of the output cycle as they do in the middle of the flow curve. At low output (typically below 1.0 to 1.4 milliseconds of pulse width depending upon the injector) and the top end (above 85 to 90%), injectors operate in a non-linear fashion. This means that the injector can no longer be trusted to deliver the exact amount of fuel that is commanded to flow. If less fuel is delivered, this can be catastrophic for engine survival.
The non-linear behaviour of injectors can occur at light throttle driving as well, depending on the injector characteristics. In general, the larger the fuel injector, the lower the pulse width needs to be to achieve the same air-fuel ratio as a smaller injector.
To allow for inherent injector variables, it is common practice to reduce the injector's duty cycle. For example, Holley recommends sizing fuel injectors for a normally aspirated mild street engine where the maximum duty cycle is limited to a maximum of 70%.
Frequently asked questions
Fuel injectors are a part of a car's engine that deliver fuel to the engine.
No, fuel injectors will not always be linear. At low output (typically below 1.0 to 1.4 milliseconds of pulse width) and the top end (above 85 to 90 per cent), injectors operate in a non-linear fashion.
Non-linear fuel delivery means that the injector will not deliver the amount of fuel that is expected. For example, if an injector is expected to deliver 40 cc/min at 1.0 ms pulse width, it should theoretically deliver 35 cc/min at 0.8 ms, but in almost all cases this is untrue.
If a fuel injector is non-linear, the fuel delivery will drastically drop off, creating a lean condition. This can cause a hunting idle or a lean misfire, which will make the engine run very choppy and unstable.
To avoid a non-linear fuel injector, it is best to avoid operating the injector in the lower and upper 5 to 10 per cent of their flow ranges. It is also common practice to reduce the injector's duty cycle, which is expressed as a percentage of operation.
