Gasoline ≈ 0.74, Diesel ≈ 0.84, E85 ≈ 0.78. Used for volume estimation.
Mass Flow Rate0 lb/hr
Flow Rate (kg/hr)0 kg/hr
Flow Rate (grams/sec)0 g/s
Volume Flow (Gallons/hr)0 gal/hr
Volume Flow (Liters/hr)0 L/hr
Min. Injector Size (4 Cyl @ 80% Duty)0 cc/min
How to Calculate Mass Flow Rate of Fuel in an Engine
Calculating the fuel mass flow rate is a fundamental step in engine tuning, fuel system sizing, and performance engineering. It determines exactly how much fuel an engine requires to produce a specific amount of power safely. Whether you are sizing a fuel pump for a race car or analyzing efficiency, understanding the mass flow rate ($\dot{m}_f$) is critical.
The Fundamental Formula
The most direct method to calculate the required fuel mass flow rate is based on the engine's power output and its Brake Specific Fuel Consumption (BSFC). The formula connects the energy demand (Power) with the engine's efficiency in converting fuel to power (BSFC).
$$ \dot{m}_f = P \times \text{BSFC} $$
Where:
$\dot{m}_f$: Mass flow rate of fuel (lb/hr or kg/hr)
$P$: Brake Power (HP or kW)
BSFC: Brake Specific Fuel Consumption (lb/hp·hr or g/kWh)
Step-by-Step Calculation Guide
1. Determine Target Power
Start with the peak power output of the engine. If you are building a new engine, use your target horsepower. For example, a turbocharged 4-cylinder engine aiming for 300 HP.
2. Estimate BSFC
BSFC measures how efficiently an engine uses fuel. Lower numbers indicate better efficiency, but high-performance engines often run richer (higher BSFC) for cooling and knock prevention.
Engine Type
Typical BSFC (Imperial)
Typical BSFC (Metric)
Naturally Aspirated (NA)
0.45 – 0.50 lb/hp·hr
270 – 305 g/kWh
Turbocharged / Supercharged
0.55 – 0.65 lb/hp·hr
335 – 395 g/kWh
Methanol / E85
0.70 – 0.85 lb/hp·hr
425 – 515 g/kWh
3. Perform the Calculation
Imperial Example:
Engine: 300 HP Turbocharged
BSFC: 0.60 lb/hp·hr
Fuel pumps and injectors are often rated in volume (Liters per hour or cc per minute), not mass. To convert mass flow to volume, you must divide by the fuel's density.
Fuel Pump Sizing: Your fuel pump must be able to supply more than the calculated maximum flow rate. A general rule of thumb is to size the pump for 20-30% more flow than calculated to account for line losses and aging components.
Injector Sizing: Once you have the total mass flow rate, you divide it by the number of cylinders and the target duty cycle (usually max 80% or 85%) to determine the required injector size in cc/min or lb/hr.
function toggleUnits() {
var system = document.getElementById('unitPreference').value;
var powerLabel = document.getElementById('powerLabel');
var bsfcLabel = document.getElementById('bsfcLabel');
var bsfcHelper = document.getElementById('bsfcHelper');
var enginePower = document.getElementById('enginePower');
var bsfcValue = document.getElementById('bsfcValue');
if (system === 'imperial') {
powerLabel.innerText = "Target Engine Power (HP)";
bsfcLabel.innerText = "Brake Specific Fuel Consumption (lb/hp·hr)";
bsfcHelper.innerText = "Typical: 0.45-0.50 (NA), 0.55-0.65 (Turbo).";
// Optional: clear values or convert them if you wanted to be fancy, but clearing is safer to avoid confusion
enginePower.placeholder = "e.g. 300";
bsfcValue.placeholder = "e.g. 0.50";
} else {
powerLabel.innerText = "Target Engine Power (kW)";
bsfcLabel.innerText = "Brake Specific Fuel Consumption (g/kWh)";
bsfcHelper.innerText = "Typical: 270-305 (NA), 335-395 (Turbo).";
enginePower.placeholder = "e.g. 220";
bsfcValue.placeholder = "e.g. 300″;
}
}
function calculateFuelFlow() {
// Get inputs
var system = document.getElementById('unitPreference').value;
var power = parseFloat(document.getElementById('enginePower').value);
var bsfc = parseFloat(document.getElementById('bsfcValue').value);
var densityKgL = parseFloat(document.getElementById('fuelDensity').value); // Always kg/L for internal calc logic simplification
// Validation
if (isNaN(power) || power <= 0) {
alert("Please enter a valid Engine Power.");
return;
}
if (isNaN(bsfc) || bsfc <= 0) {
alert("Please enter a valid BSFC value.");
return;
}
if (isNaN(densityKgL) || densityKgL kg/hr = (kW * BSFC)/1000
flowKgHr = (power * bsfc) / 1000;
// Convert to lb/hr: 1 kg = 2.20462 lb
flowLbHr = flowKgHr * 2.20462;
}
// Derived units
var flowGs = (flowKgHr * 1000) / 3600; // grams per second
// Volume Flow Calculation
// Volume L/hr = Mass kg/hr / Density kg/L
var volLHr = flowKgHr / densityKgL;
// Volume Gal/hr
// 1 Gallon (US) = 3.78541 Liters
var volGalHr = volLHr / 3.78541;
// Injector Estimation (Generic 4 cylinder estimation for context)
// Total flow (cc/min) = volLHr * 1000 / 60
var totalCcMin = (volLHr * 1000) / 60;
// Per cylinder (4 cyl assumption) at 80% duty cycle
// Required Capacity = (Flow per cyl) / 0.80
var estInjectorSize = (totalCcMin / 4) / 0.80;
// Display Results
document.getElementById('results').style.display = 'block';
if (system === 'imperial') {
document.getElementById('mainFlowResult').innerText = flowLbHr.toFixed(2) + " lb/hr";
} else {
document.getElementById('mainFlowResult').innerText = flowKgHr.toFixed(2) + " kg/hr";
}
document.getElementById('flowKgHr').innerText = flowKgHr.toFixed(2) + " kg/hr";
document.getElementById('flowGs').innerText = flowGs.toFixed(2) + " g/s";
document.getElementById('volGalHr').innerText = volGalHr.toFixed(2) + " gal/hr";
document.getElementById('volLHr').innerText = volLHr.toFixed(2) + " L/hr";
// Rounded to nearest 10 for injector
document.getElementById('recInjector').innerText = "~" + Math.round(estInjectorSize) + " cc/min";
}