Written by Brad Shantry
Everyone has heard of these 1000+ horsepower street machines, just add race fuel and turn up the boost. With the supercharger and turbocharger technology of today combined with a stout long block the only real limiting factor to 1000+ horsepower street cars is fuel. Sure a methanol kit can add some much needed octane, but is far from an ideal scenario. A more advanced option has been to install 2 fuel tanks, and manually switch between them, which typically means, shutting off the engine, turning up the boost and either mechanically or electrically switching which fuel tank feeds the fuel rail.
Imagine if you could just take off in your boosted street machine on an event like the Hot Rod Powertour or Drag Week and have the big power, just because you put the hammer down, but not have to worry about finding a place with expensive race fuel every couple of hundred miles? The time is now, with the right EFI controller and some added hardware, you can be driving down the highway on premium pump gas, bury your right foot and have all the power you can handle.
EFI BASICS
To understand how it works, we need to first review how multi-port electronic fuel injection works. Contrary to the name a fuel injector is a solenoid valve, it does nothing to ‘inject’ the fuel. When the Engine (or Power) Control Module (ECM) sends an electric charge to the fuel injector, it open and allows pressurized fuel to flow thru and atomizes it due to the nozzle design.
The percentage of fuel which flows thru the injector is based on how long the PCM sends electric charge holding the injector open, the process is referred to as Pulse Width Modulation (PWM). As the amount of fuel consumed by the engine increases the pulse width gets longer. As the engines rpm’s increase the amount of time the fuel injector can remain open per combustion cycle decreases. The duty cycle of the fuel injector at any given time is its pulse width divided by the time available between combustion events. 100% duty cycle means that the pulse width has reached the amount of time available between combustion cycles; therefore the fuel injector is receiving a constant electrical charge and remains open.
The curve ball to all of this is that pulse width does not actually control the maximum amount of fuel an injector flows, rather the percentage of its maximum flow. The maximum flow rate of the injector is limited by its internal dimensions. Fuel injectors are rated by that maximum mass flow rate of fuel at 100% duty cycle and a given fuel pressure. The general rule of thumb when choosing an injector is to purchase a set which is rated just higher than the maximum amount of fuel the engine is expected to be capable of consuming. By running the smallest available injector, pulse width at idle is greater than if a larger fuel injector was chosen. The larger pulse width at a given RPM, means the ECM can make smaller changes to pulse width to control idle, creating a higher idle quality.
Traditional EFI
OCTANE ON DEMAND
The octane on demand system works much the same as the new displacement on demand systems OEM’s are using for fuel economy. In displacement on demand systems, the ECM deactivates cylinders which are not required to meet the vehicles instantaneous power requirement. In reality the power required to increase a vehicles speed is far greater than the power required to maintain it. The octane on demand system does not deactivate cylinders, but rather fuel injectors which are dedicated to high octane fuel.
FIGURE 2 - OOD in normal cruising operation
The cross section of a traditional multi-port fuel injection system looks something like figure 1. There is only one type of fuel, and one injector per cylinder; therefore typically one fuel tank, pump, filter and regulator supplying the fuel rail(s). With the ‘octane on demand’ system an entire second fuel system is required including fuel tank, filter, pump(s), regulator and rail(s). Also another fuel injector is required for each cylinder The ECM is the key to the system; it must be capable of controlling the fuel injectors which are supplied with premium pump fuel (primary injectors), and controlling the fuel injectors which are supplied with high octane fuel (secondary injectors). At Wheel to Wheel Powertrain we use the Big Stuff 3 ECM to accomplish it. The way the Big Stuff 3 controls the extra fuel injectors is quite simple. During typical operation it sends a pulse width to only the primary injectors (see figure 2). It will continue functioning that way until the primary fuel injectors reach a duty cycle which the calibrator specifies (typically 80%). At that point the ECM splits the pulse width between the primary and secondary injectors (see figure 3). The new pulse width is calculated by the ECM based on the maximum flow rate of the primary and secondary injectors. It then sends the same pulse width to all of the injectors; therefore the percentage of high octane fuel is controlled by the ratio of maximum flow between the primary and secondary fuel injectors, the result is the effective octane rating, see the following equation.
OOD Formula
This system creates four variables which mean several things to consider when choosing injector sizes and fuel types. Here are just some of the considerations which need to be in determining the right combination for any ‘octane on demand’ application:
* How much octane does the engine combination require? The effective octane obviously must be set higher than the required octane.
* As discussed earlier, in terms of idle quality a smaller primary injector is more desirable.
* A smaller primary injector compared to the high octane injector, increases the effective octane rating, which also means the rate of high octane fuel consumption increases.
* The maximum flow rate of the primary and secondary injectors must be at least the amount of fuel required to support the engines maximum power.
* The primary fuel injector needs to be capable of supporting the vehicle at cruising speeds.
FIGURE 3 - OOD During high octane operation
OK let’s get real and set up a system for a 1000 horsepower turbocharged 427 cubic inch engine. Let’s assume that we need an effective octane rating of at least 100 octane. There are several formulas for the maximum fuel flow to support that power level, 80lbs/hr should be more than sufficient for this example. Let’s look at using 24lbs/hr primary and 60 lbs/hr secondary injectors. The combined maximum fuel flow is 84lbs/hr, which is greater than the 80lbs/hr that we determined would be required. The 105 effective octane rating for this combination is greater than the required 100 octane rating we determined was required previously. Further more the 24lbs/hr primary fuel injector should provide a good idle quality. The final consideration is that a 24lbs/hr fuel injector in an 8 cylinder engine is capable of supporting over 300hp. A heavy car with poor aerodynamics requires less than 50 horsepower to maintain a 75 mph cruise. Which means the primary injector will allow support the majority of driving situations, without using the precious high octane fuel. It appears as though we have chosen a good combination of fuel injectors and octane requirements for our example application.
example
ADDITIONAL CONSIDERATION
There are several other things to consider when developing an octane on demand system for your vehicle.
* All of the additional components required for an ‘Octane on Demand’ system require significant fabrication and planning.
* Mounting the additional fuel injectors involves several considerations. Most importantly there is not a good means for installing them into the runners of a composite intake manifold.
* If the primary fuel injectors are sized relatively small, it is difficult to have a situation where an engine produces enough boost at low rpm’s to create knock without exceeding the primary injector’s capability, but it is theoretically possible. It is important to adjust the duty cycle which engages the secondary injectors to avoid such a knock situation.
* The secondary fuel system needs to remain pressurized, but will during most driving conditions will not be allowing fuel to flow into the engine. Constant pressure without flow can cause premature fuel pump failure. Also allowing fuel to stagnate in the fuel rail during operation is likely to cause vapor lock. There are several options and rationales for coping with these issues, consider all options carefully.
* When building the secondary fuel cell it is important to consider what volume is needed. There is not a hard fast rule for tank sizing, but you can calculate consumption rates and decide how many minutes of fuel you need at maximum flow. On average high octane gasoline weighs about 7 lbs/gallon. Keep in mind this equation is based on the secondary fuel injector at 100% duty cycle. In reality the system is designed to use less than 100% duty cycle of the secondary fuel injector. In an acceleration situation, the fuel injectors duty cycle will vary with engine RPM, therefore the secondary fuel consumption is less than 0.875 gallons/minute.
Fuel Consumption Calculations
* Finally, tuning big power engines, mistakes turn catastrophic quicker and typically cost more. If you are not experienced in tuning, it is a good idea to have the system professionally tuned.
Check out the video run down of the system on Powertv
