Air-fuel ratio: Precise fuel delivery required for power and economy
Air-fuel ratio is another key engine control component. The proportional mass of air and fuel within this mixture is critical for controlling burn rate, power extraction, and fuel economy. Performance engine calibration, as it relates to fuel control, is divided into two distinct objectives: fuel economy and maximum safe power. OEM calibrations also take emissions into account. For the best fuel economy, the focus is on having enough air present to ensure that all of the fuel is combusted. In many cases, air-fuel ratios close to stoichiometric will be ideal. Stoichiometric air-fuel ratios are those that follow the exact mass ratios based on the chemistry of the fuel used. In this way all of the fuel is burned by the available mass of air and this approach is consistent for all fuel types.
When it comes to producing the peak amount of safe horsepower, the goal is different. In this case, the objective is to choose an air-fuel ratio that ensures that all of the air is used up in the combustion process. For this to occur, a fuel rich mixture is introduced. Richer fuel mixtures slow down the combustion process and create a controllable burn rate. The additional fuel also serves to cool the cylinder and thus preserve power and control combustion under high load.
With gasoline, stoichiometry is achieved at a 14.7:1 air-fuel ratio (AFR). A high load condition will often require an AFR between 10.5:1 and 12.5:1 depending upon fuel quality and engine type. The exact and appropriate AFR required for each fuel and engine is unique. Timing, fuel, and boost vary together with load, engine RPM and a vast array of other parameters to create the best overall engine instruction set.
Precise fuel delivery allows lean and economic conditions under low load and richer conditions under boost and high load. The ideal AFR varies with fuel type and in concert with ignition timing, engine speed, and boost pressure. The ideal set of engine instructions are interdependent and complex.
Boost: Too much of a Good Thing?
Manufacturers spend a tremendous amount of time choosing turbochargers that best match the pumping characteristics of the engine. Displacement, desired power, responsiveness, and fuel quality are the driving forces behind turbo choice. Most original equipment turbochargers are small relative to the engine’s displacement to enhance responsiveness or “spool-up.” This provides excellent drive quality but provides little opportunity to create unlimited power.
As air is compressed it becomes hotter. This expected temperature change is described by the ideal gas law which relates temperature, volume, pressure and mass. Factory turbochargers are most efficient at lower boost. When boost pressures are increased beyond factory levels, the outlet temperatures from the compressor increase. Part of the temperature increase is natural and expected. However, there will be additional heat created when the turbocharger goes beyond its efficiency range. A turbo is only “efficient” when just a small fraction of heat is added to compressed air beyond the increase predicted by the ideal gas law. The heat that results from compressing air is the reason that most turbocharged cars are intercooled. The intercooler lowers the charge air temperatures to create a more controlled combustion process.
The ideal boost limit for a given turbo is a function of its compressor efficiency. When boost pressures are kept within the efficient range of the turbo, turbo outlet temperatures stay under control and the relatively cool air entering the motor is a part of a controlled combustion process that promotes power. Through practical experience we have found that mid-RPM boost pressures of more than 19 psi on stock R35 turbochargers produces no additional power. Likewise, boost pressures at a modest 15 psi at higher RPM are just within the efficient range and flow capabilities of these stock turbos. Because of this narrow window of turbocharger efficiency, we chose to use similar and modest boost levels for all of the fuels tested.
91 Octane: Kalifornia Crude
We first tuned the DSPORT R35 with standard California 91 octane pump fuel. California 91 octane is famous for its poor quality. Throughout the tuning world, it’s well understood that it’s hard to make power on lovely California pump fuels. Why is California 91 so poor compared to other fuels with similar octane rating? You can thank the California government entity for mandating the special mixture of hydrocarbons, cleaning agents, and ethanol that performs so poorly. While their intentions are certainly noble, the result is a 91-octane fuel that burns and reacts like a much lower octane mixture. This brings us to the defining point of this particular fuel: low octane. Lower octane fuels often burn more quickly and less controllably than higher-octane pump fuels. The faster and less predictable burn rates of 91-octane pump fuel meant that we used as much as 2 to 5 degrees less ignition timing compared to 100-octane and E85 (105-octane equivalent). In addition, we were forced to run comparably richer AFR. Lower ignition timing and richer fuel mixtures are used with this fuel to control the relatively volatile burn characteristics of 91-octane fuel. However, these same changes to accommodate the poor fuel create less overall mechanical torque and thus, less power. Because we used similar boost pressures across the three fuels its easy to see the relatively large impact of this requisite conservative tuning approach. The DSPORT R35 created significantly less power and torque compared to the higher octane fuels tested here (550 whp and 540 lb-ft torque).The Kalifornia Crude 91-octane pump gas is indeed a compromise that leaves power on the table in order to avoid detonation and engine damage.
100-octane unleaded: Expensive, but worth it
Immediately after finishing the 91-octane tune, the DSPORT crew went to work and drained the tank on Project R35 (see sidebar for proper tank draining technique). The tank was then refilled with VP Racing Fuel’s VP 100 unleaded race fuel. Without changing the engine calibration (still set for 91 octane) we made our first pulls on the dyno. Not surprisingly, our Project R35 actually LOST power when we changed to race fuel. Why? Our calibration was set up for a fast burning, low-octane fuel. This was completely inappropriate for 100-octane unleaded race fuel. The 100-octane unleaded race fuel is much less detonation prone, burns more slowly, and has a slightly higher specific gravity. As a result, the ideal calibration for 100 octane required much higher ignition timing (red, third panel of figure 3) and a leaner AFR. Together these calibration changes with little change in boost pressure increased peak torque by 50 lb-ft while peak power increased by 35 horsepower.
E85: Race Gas At the Pump
A relative new arrival to the high performance fuel world is the ethanol based biofuel known as E85. As the name implies, E85 is 85-percent ethanol and 15-percent gasoline. E85 is available as an alternative pump fuel in many parts of the US. The genesis of E85 is linked to a complex political attempt to reduce America’s dependence on foreign oil. To what extent this objective has succeeded or failed is the source of much debate but there is absolutely no debate in regards to the performance potential of E85. Its octane (105), cooling properties, oxygen content, and availability make it the most popular high performance fuel. E85 is effectively race fuel available at the pump for less than the price of regular unleaded.
E85 has a very different chemistry compared to 100-octane and 91-octane gasoline and its stoichiometry represents a blend of these properties. Because of these differences we could not test E85 using the 100-octane calibration. We first had to change the calibration to account for the fact that E85 requires approximately 30-to-35 percent more fuel. After making these basic fuel delivery changes the car was tested repeatedly to determine the best ignition timing, fuel, and boost control. E85 is high octane, burns in a very predictable and slow manner, and has enhanced detonation resistance brought on by the extra cylinder cooling effect created by 30-percent greater fuel volume. As you might predict, ignition timing was increased even further (Figure 3, panel 3) and the fuel mixture was moved toward ideal for power at a gasoline AFR equivalent of about 12.0:1 (Figure 3 panel 4). With ideal timing and leaner AFR the DSPORT R35 picked up an amazing 50 wheel horsepower and 50 lb-ft of torque compared to 100-octane race fuel. Compared to 91-octane, peak power and torque were up 90 horsepower and 90 lb-ft of torque without significant changes in boost pressure (Figure 3, panel 1).
The octane and cooling qualities of E85 do not fully explain the huge increase in power. The substantial increase in power is octane along with oxygenation and fuel mass. The chemistry of E85 requires more fuel mass for each mass of air and this fuel mass contains huge amounts of oxygen.