FUEL INJECTION

ELECTRONIC FUEL INJECTION (EFI)

by Double H

If there's one thing that's critical in a high performance engine, then it's fuel control. Think about it: the whole objective of adding a turbocharger, of installing NOS, even of installing a free flow exhaust system, is to improve fuel delivery into the combustion chamber. It is also events in the combustion chamber that can and will destroy a high performance race engine if it's not controlled properly. Here we're talking about controlling the combustion process. Now I've heard many arguments as to why sidedraft carburetors provide better performance than fuel injection and engine management, and vice versa but I always say: it's not about performance, it's about reliability and there's no better system for fuel control than electronic fuel injection. Any endurance race car from INDY Car Racing, to Formula 1, to the World Rally Championship, to the Le Mans Series uses electronic fuel injection (EFI) systems, not just for reliability but because ensuring that the correct amount of fuel is delivered under every condition, will provide the best performance.

EFI is central to engine management. It relies on an engine control unit (ECU) which processes a number of inputs from various sensors on the engine to deliver the correct amount of fuel at a particular RPM and air-flow rate/air density combination. The fuel is delivered through an injector, which is an electronically actuated solenoid valve. The amount of fuel that is delivered is dependent on the fuel pressure, which is usually a constant 30 psi above intake manifold pressure, and the pulse duration of the injector, i.e., the length of time the injector is held open.

Most EFI systems have a standard set of sensors. These include:

• The Barometric Pressure (BARO) Sensor, which provides the ECU with the atmospheric air pressure reading.
• The Engine Coolant Temperature (ECT) Sensor, which provides the ECU with the engine's current operating temperature. This is important because fuel vaporization varies for different engine temperatures. A cold engine requires more fuel while a hot engine requires less.
• The Intake Air Temperature (IAT) Sensor, which the ECU needs to take into account when determining pulse duration.
• The Mass Air Flow (MAF) Sensor, which is a tube positioned after the air filter in the air intake duct. The MAF sensor has a fine platinum wire that spans across the tube. The wire is heated by electrical current to maintain a constant temperature above ambient. The air flow past the wire cools the wire and more current is required to maintain the constant temperature. Thus, the amount of current required to maintain the constant temperature indicates the air flow rate. The air flow rate is divided by RPM to determine the pulse duration.
• The Manifold Absolute Pressure (MAP) Sensor, which uses manifold vacuum to measure engine load. An EFI system that uses a MAP sensor does not require a MAF sensor as it can use the input from the MAP sensor to determine the required pulse duration.
• The Oxygen Sensor (O₂S), which is used to measure the amount of oxygen that is not consumed during combustion. This is important for the correct operation of the catalyst converter and is used for emissions control rather than performance or economy. The O₂S is located in the exhaust system and is an after-the-fact measure of the air/fuel ratio. Too much unburnt fuel in the exhaust indicates a lean mixture while too little oxygen indicates a rich mixture.
• The Crankshaft Position (CKP) Sensor, which is important for timing purposes as it tells the ECU which spark plug to fire and which injector to open at any given point in the Otto cycle.
• The Throttle Position (TP) Sensor, which is another important sensor as the throttle position and the rate of change in the throttle position indicates the what the diver wants the car to do.

The modifications you can perform on an OEM EFI are somewhat limited because the OEM ECU is not reprogrammable. On older model cars there are a number of things you can do to modify the EFI system without having to reprogram the ECU. You can increase the fuel pressure as this is one reading that the ECU does not take into account – it assumes the fuel pressure is a constant 30 psi above intake manifold pressure; you can intercept the pulse signal form the ECU, alter it using input from the manifold pressure and send it to the injector; you can increase the injector nozzle size; or you can increase the number of injectors. In the next few pages we'll discuss each of these options.

On modern cars the reliance on the ECU to ensure that you can produce as much horsepower as possible while retaining reliability is much greater. Even on older cars, your best option to obtaining maximum performance is to install an aftermarket ECU from companies such as Motec, Pectel, Hondata, AEM, and Cobb Tuning, and to shell out a bit on some ECU tuning.

FUEL INJECTORS

A stock injector.

There are a few important factors that you must take into account when modifying an electronic fuel injection engine. These are: the pulse duration of the injectors and the duty cycle.

The injector pulse duration is the amount of time that the injector is held open so that it can inject fuel into the combustion chamber. The pulse duration is controlled by the engine control unit (ECU) and is dependent on various sensors in the electronic fuel injection (EFI) system. The longer the pulse duration, the more fuel is added to the air/fuel mixture. The amount of fuel required at any one time varies by the amount of air flow, the air density, the engine load, and the engine temperature. Therefore the pulse duration will vary. However, there is only a limited amount of time that the injector can be held open at each revolution of the engine. This amount of time is reduced as engine speed increases. For example, at 600 RPM the available time is 0.1 seconds (60 seconds in a minute divided by 600 revolutions) but at 6,000 RPM it is only 0.01 seconds. The pulse duration relative to the available time at the engine red line is called the duty cycle and is expressed as a percentage. Thus a duty cycle of 80% means that at the engine red line the pulse duration (the amount of time the injector is help open) is 80% of the available time.

INCREASING THE DUTY CYCLE

Some engine tuners will tell you that if your car has a duty cycle of 80%, you have a possible gain of 20%. However, the injector is an electronic solenoid and cannot be held open for too long or it will overheat and fail. In practice most Nippon Denso and Rochester injectors will remain reliable at up to an 80% duty cycle; most Bosch injectors will remain reliable at up to an 85% duty cycle; and most Lucas injectors will remain reliable at up to a 92% duty cycle. Though even at these duty cycles it is still advisable to test the injectors. Test them specifically for their spray pattern and their flow volumes at the maximum duty cycle you require.

On a race engine I wouldn't exceed a maximum duty cycle of 80% as dyno-testing on various have shown that a duty cycle in the region of 60% to 70% produces the best power. This is because a shorter duty cycle does not allow for the proper atomization of the fuel, and proper atomization is important for the proper burning of the air/fuel mixture. I usually aim for a duty cycle at the point where maximum power is reached so as to ensure the longevity and reliability of the injectors. So, how do you adjust the duty cycle? By adjusting the fuel pressure and the injector nozzle size. On an OEM EFI system there are certain limitations on increasing the fuel pressure and the injector nozzle size. I'll discuss increasing the injector size in a while, but you can read more about increasing fuel pressure here.

INCREASING INJECTOR NOZZLE SIZE

Increasing the injector nozzle size will result in increased fuel delivery all the time. As "Langer" mentioned in engine basics, a rich fuel mixture results in power loss. Therefore, increasing the nozzle size could have a negative effect on performance and economy. The oxygen Sensor (O₂S) will correct the fuel mixture for an injector that is about 20% larger than stock. However, on a pre-1996 EFI system, the ECU will ignore the O₂S sensor under full throttle conditions. Furthermore, the OEM ECU will not be able to handle an injector that is more than 20% larger than stock and will suspect that one or more of its sensors are faulty and will revert to its programmed settings, which means that the fuel air mixture will not be optimal and will probably be rich as this is the failsafe setting on the ECU when it suspects that its sensors are faulty.

INCREASING THE NUMBER OF INJECTORS

Another option is to increase the number of injectors. You can do this by adding auxiliary injectors that are controlled by a separate ECU, or by adding a second ECU to control both the existing injectors as well as the additional injectors, leaving the OEM ECU to control the other engine management functions. The latter is called "staged" injection and requires an extra injector for each engine cylinder. With auxiliary injectors you don't need an extra injector per cylinder; instead you can place one or two extra injectors upstream in the intake path. To achieve the best performance, you should install the extra injectors ahead of the throttle body in the intake path as this allows for better fuel distribution and aids fuel vaporization. However, with positive-displacement superchargers, such as Roots, Eaton, or Lysholm superchargers this is not possible because the throttle body must be placed ahead of the supercharger.

While the additional ECU in both systems are fully programmable, and while staged injection is more expressive and a bit more complicated than auxiliary injectors, it does give you far more tuning control and better fuel distribution as you have full programming control over both sets of injectors. Usually, the new injectors are larger than the stock injectors and become the primary injectors with the stock injectors becoming the secondary injectors. The secondary injectors only come online at high engine loads, when extra fuel in required. This setup allows for greater tolerance of minor variations in the air/fuel mixture, and a smoother transition when the secondary injectors are in use. Whenever I need to run larger injectors, and when my budget allows it, I'd go for staged injection every time.

PERFORMANCE ECU CHIPS

Reprogrammed performance chips.

Replacing the stock ECU chip with a reprogrammed performance chip is a good option for anything up to a 10% increase in engine power. This is mainly due to the way in which the stock ECU chip is programmed. In essence, the stock ECU is programmed for optimal performance at peak torque so as this ensures that the car is drivable at low engine speeds. As most cars would be driven at low engine speeds of up to 3,500 RPM, ensuring that the car drives perfectly at these engine speeds is perfectly reasonable; unless you want to modify your car! A reprogrammed performance chip will be programmed for optimal performance up to the engine red line, releasing a moderate power increase but making the car less drivable at lower engine speeds.

This is all good and well if the performance chip is fitted on its own; however, fitting a reprogrammed chip to a modified engine for the purpose of accommodating the engine's new fuel requirements is a completely different story. The modifications carried out on your engine will alter the performance characteristics of the engine, as well as its fuel requirements and would require a chip programmed to meet those specific requirements. Thus, the chip will be specific to your car and could be quite expensive. Fortunately, some turbo and supercharger kit manufacturers will include bigger injectors and a reprogrammed chip in their kit but they would assume that you have made and will make no other modifications to your engine, or the reprogrammed chip will be useless.

Should you want to carry out additional modifications to your engine, you would need a different reprogrammed chip as you engine characteristics would change. Therefore, before you consider getting a reprogrammed performance chip for your ECU, you should first consider whether you will want to carry out more engine modifications at a later date. In such an event, it may be a better idea to fit a fully programmable aftermarket ECU that will allow you to reprogram the ECU as you make modification changes to the engine.

FUEL PRESSURE AND RISING-RATE PRESSURE REGULATORS

A rising-rate fuel regulator from BEGi.

On forced induction cars that use electronic fuel injection, you can use a rising-rate fuel pressure regulator to increase the fuel pressure proportional to the increase in boost pressure. The rising-rate fuel pressure regulator was developed by Ron Nash in the mid 1970's and has two adjustments: a needle-valve side adjustment that sets the maximum fuel pressure at maximum boost pressure, and a spring loaded onset screw that sets the boost pressure at which the fuel pressure will begin to increase. Note, however, that the rising-rate fuel pressure regulator does not replace the stock fuel pressure regulator but is used together with the stock fuel regulator. Using a rising-rate fuel pressure regulator allows you to retain the stock injectors but your fuel pump must be able to provide the required fuel pressure. It is advisable that the fuel pump should supply about little more than the maximum required fuel pressure. If your fuel pump does not provide sufficient fuel pressure, you will need to install a high-pressure fuel pump or a second fuel pump.

Using a rising-rate fuel pressure regulator allows you to maintain EFI timing and fuel delivery relative to signals from the air flow meter. Unfortunately, there are some limitations to using rising-rate fuel pressure regulator. Firstly, your stock fuel pressure regulator must be able to handle the increased pressure without its diaphragm failing. The stock fuel pressure regulator can usually handle up to 100 psi of fuel pressure so anything above 100 psi would become a problem. Fortunately, the fuel lines on an EFI equipped engine can safely withstand pressures of over 100 psi so there will be no need to replace them; but they should be checked for weaknesses. Secondly, and more importantly, the stock fuel injector will only operate correctly at up to 60 psi. Once the fuel pressure rises above 60 psi, the longevity of the injector will be greatly reduced. In addition, injector control will become quite erratic once the fuel pressure rises above 70 psi. Thus, this method of increasing fuel delivery is only practical for mild boost pressures of up to 9 psi. If you intend running higher boost pressures, you should consider installing a fully programmable aftermarket ECU that will allow you to program the ECU for the correct fuel delivery at different boost pressures and throttle positions.

FUEL PUMPS AND THE FUEL SUPPLY

A high-pressure roller-type electronic fuel pump is required to supply the fuel and the fuel pressure to the fuel rail and the injectors. The fuel pump must be large enough to supply more fuel than the maximum amount that the engine may require to ensure that the fuel pressure remains adequate at full throttle and at maximum RPM. Remember that when you alter the fuel pressure regulator or use a risings rate fuel pressure regulator to increase the fuel pressure in your EFI system, you also need to increase the fuel flow rate or fuel flow volume of your fuel pump as it has to pump against higher pressures. When the pressure increases, the fuel pump operates against more resistance and hence it operates at a slower pace. However, make sure that you don't increase the fuel flow rate too much or else the fuel will be traveling to the fuel rail and back to the tank too often and will pick up heat. Heat has a detrimental effect on the octane rating of the fuel! Therefore you should not exceed the required fuel flow rate by more than 33%. You can use the formula hp × 7.3 to find the required fuel flow rate in cubic centimeters (cc) per minute that will give you a 33% over supply of fuel.

INCREASING THE FUEL FLOW RATE

You can increase the fuel flow rate by installing a larger fuel pump or by installing a second fuel pump. Installing a second fuel pump is the best solution for a modified street car as it allows you to wire the second pump separately so that it is only in use when extra performance is required. More often than not you'd be using a street race car for cruising. Under these conditions you would not need a high fuel flow rate, in fact, it would be best not to have a high fuel flow rate under these conditions as it will just increase the amount of fuel that circulates to the fuel rail and back to the tank. This will expose the fuel to heat, which is not good. You can wire the second pump to only operate when it is required by installing a pressure sensitive switch on the intake manifold, or by installing a microswitch on the throttle body.

The two pumps can be installed in parallel with each pump having its own fuel pickup, or in series with one pump pumping into a second pump. When you install two pumps in parallel, you increase the fuel flow rate; but the two pumps must be of equal capability. Here the fuel flow rate will be the sum of the fuel flow rate of the two pumps. When you install tow pumps in series, you increase the fuel pressure rather than the fuel flow rate. This is because the second pump is being force fed by the first pump and fuel pressure is increased between the two pumps. Hence the second fuel pump does not need to increase the fuel pressure by much to meet the fuel pressure set by the fuel pressure regulator. However, the fuel flow rate does not increase by much and will only be approximately 120% of the fuel flow rate of the largest pump.

THE FUEL PICKUP

The fuel pickup should also get some consideration, especially on a modified street race car with a steel fuel tank. On such vehicles the fuel level in the fuel tank can drop below the fuel pickup during hard cornering or heavy acceleration and heavy braking and can cause a momentary drop in fuel pressure and engine performance. The solution is to have a secondary fuel pump supply a small fuel reservoir or swirl pot that feels the primary fuel pumps. The fuel reservoir is a simple cylinder with an fuel inlet from the secondary pump at the side, a fuel return line at the top and fuel pickup lines to the primary pumps at the bottom. The fuel reservoir should have a capacity of at least 1 liter and should be mounted low, beside the fuel tank. With a fuel reservoir, the primary fuel pumps will always have a fuel supply even under hard cornering of heavy acceleration and braking conditions.

(Credit to Custom Car US)

Posted in: Information