ADDING FUEL TO THE FIRE! Determining Fuel System Requirements
ADDING FUEL TO THE FIRE! Determining Fuel System Requirements
WHAT IS BSFC?No conversation about fuel system requirements would be complete without addressing BSFC. BSFC stands for Brake Specific Fuel Consumption and is used to determine how much fuel your engine will use for each horsepower it makes. It can be used to determine fuel injector and fuel pump sizing, but most tuners use BSFC after the fact to validate the system capabilities on the dyno to improve efficiency if BSFC is high. Summit Racing provides a chart with BSFC by fuel type here:https://bit.ly/summit_racing_bsfc_chart. We do it a little differently though…
STEP 1: DETERMINE YOUR HP GOALStart by determining your power goal and the airflow required to achieve that goal. 1lb/min airflow to make approximately 9 BHP So, divide power goal by 9 to determine airflow requirement in lbs/min and multiply by 60 to get lbs/hr, then divide by fuel stoichiometry *lambda target to get fuel flow requirement (it’s easier than it sounds!!) THE FORMULA: [FuelFlowRequired] = (([HP Goal] ÷ 9) X 60) ÷ ([FuelStoich] X [LambdaTarget])
PUMP SELECTION:-Pump pressure - Confirm that the pump can supply your required flow rate at the expected working fuel pressure. In the chart below to the right, the high flow pump (green trace) will flow approximately 175 LPH and consume 10 A of current at 70 PSI. By comparison, the factory fuel pump flows approximately 80 LPH at the same working fuel pressure. -Boosted vs NA - Expected fuel pressure for boosted applications will be base fuel pressure, which is mechanically set by the regulator, plus boost pressure. If your base fuel pressure is 43 PSI and boost pressure is 20 PSI, your working fuel pressure will be 63 PSI. In an NA application, the working fuel pressure IS the base fuel pressure and it remains static. -Pump flow rates - Most high-flow fuel pumps advertise their flow ratings at 40 PSI, which is the industry standard. However, base fuel pressure varies by engine, so it's important to know what the fuel pump pressure is at the base fuel pressure of the engine in your vehicle to determine accurate flow rates.
LINE SIZING:-Friction between the fuel and inner walls of the fuel lines affects pressure at the regulator, so correct fuel line size is important -Line size affects pressure at the rail, so it will play a role in how large of a pump to select Formula for Determining Line Sizes:
FUEL PRESSURE REGULATORS:Injection pressure vs. fuel pressure – Injection pressure is the differential pressure between the intake manifold and pressure in the fuel rail. Pressure in the rail pushes the fuel out of the injector, and pressure or vacuum in the manifold can either pull fuel out or fight against the pressure behind the injector, it’s like trying to walk through a doorway when someone is trying to come out at the same time. Injection pressure is a factor in the final flow rate of the injector. KNOW WHAT YOUR INJECTORS FLOW AT YOUR REQUIRED INJECTION PRESSURE! Line size considerations – Don’t skimp on the return line! It’s like skipping leg day (put down the luggage). Seriously, ideally, your return line size should be the same as the feed line, or at minimum one size smaller only. This is necessary to accurately set your base fuel pressure. A regulator works by bleeding off excess fuel volume to regulate pressure and if that volume does not return fast enough you will end up with a false ‘high’ base fuel pressure. Rising rate vs non-rising rate – A rising rate fuel pressure regulator is used in boosted applications. It allows for static injection pressure, whereas a non-rising rate regulator that is typically used in NA vehicles will result in an increase in injection pressure under vacuum. Again, think about trying to go through a door one way when someone is coming out the other way. A rising rate regulator increases fuel pressure behind the injector to compensate for boost in the intake manifold that is pushing back against the fuel trying to come out of the injector, maintaining a static injection pressure.
FUEL FILTERS:Selecting an element type - Consider your fuel type when selecting a fuel filter element, and make sure it is rated for the type of fuel you are running, especially if you are using alcohol (ethanol or methanol) fuels. There are cellulose replaceable high-flow elements designed for alcohol fuels, and stainless-steel versions are available, although they do not, generally, filter out smaller particles like cellulose types. Surface area – Research the surface area of the filtration media. The goal is to get a surface area as big as possible to minimize clogging. Ease of use - Filters can get expensive, but high flow filters are available that have race-ready enclosures and replaceable filtration elements. If you purchase one of these, make sure that the gaskets and materials are suited for alcohol-based fuels if you are running them to prevent le Micron ratings - what does it mean? - A micron is a micrometer, which is a thousandth of a millimeter. The micron rating is the size at which particles are retained by the filter. For example, a seven-micron filter will stop particles of seven microns or larger from passing through the media. Pay attention to a filter's flow rating when considering its filtration capabilities (this is where surface area and good engineering are your friends). Flow considerations - Make sure that the filter has the flow capabilities your fuel system requires at the fuel pressure your vehicle is operating at. Too small of a filter will affect flow and pressure, and will clog quickly, which sucks when you make it to the final round only to DNF because the engine can't get fuel. It's happened.
- Preferred style for performance applications
- Mitigates issues like picking up air in the system
- Avoids vaporlock
- Seriously, use this design!
FUEL SYSTEM DESIGN - Return-Style Fuel System:
- Typically found in modern OE applications
- Easier packaging
- Generally lower cost
- Susceptible to vapor lock
- Air past the regulator must pass through injectors
- Not recommended for most performance applications