Engine Mass Flow Rate Calculator
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Engine Mass Flow Rate Calculator
Understanding Engine Mass Flow Rate
The Engine Mass Flow Rate Calculator is an essential tool for automotive engineers, tuners, and enthusiasts looking to optimize internal combustion engines. Mass flow rate ($\dot{m}$) represents the actual mass of air entering the engine per unit of time. Unlike volume flow, which changes with temperature and pressure, mass flow is directly proportional to the fuel required for combustion, making it the critical metric for ECU tuning and turbocharger selection.
How the Calculation Works
This calculator utilizes the Ideal Gas Law combined with engine geometry to determine the mass of airflow. The core formula used is:
Mass Flow = Volume Flow × Air Density
1. Determining Volume Flow
First, the calculator determines how much volume the engine displaces per second based on RPM and displacement:
- 4-Stroke Engine: Air is drawn in once every two revolutions.
- 2-Stroke Engine: Air is drawn in every revolution.
- Volumetric Efficiency (VE): This corrects the theoretical displacement. A naturally aspirated engine might have 85-95% VE, while forced induction (turbo/supercharger) engines can exceed 100% VE.
2. Calculating Air Density
Volume alone doesn't tell us how much oxygen is available. We must calculate air density ($\rho$) based on:
- Manifold Pressure: Higher pressure (boost) increases density.
- Intake Temperature: Cooler air is denser, which is why intercoolers are used in performance applications.
Why is Mass Flow Rate Important?
Turbocharger Selection
Turbo compressor maps are read using Pressure Ratio on the Y-axis and Mass Airflow (usually in lb/min) on the X-axis. By calculating your engine's required airflow at a specific RPM and boost target, you can select a turbo that operates in its peak efficiency island.
Fuel System Sizing
To maintain a specific Air-Fuel Ratio (AFR), the fuel injectors must supply a mass of fuel proportional to the mass of air. For example, at a 12.5:1 AFR, for every 12.5 lbs of air, you need 1 lb of fuel. Knowing the air mass flow allows for precise injector sizing.
Common Unit Conversions
Tuners across the world use different units. This calculator provides the most common ones:
- g/s (Grams per Second): Common in modern ECU data logging (OBDII PID).
- lb/min (Pounds per Minute): The standard unit for reading Turbocharger compressor maps in the US.
- kg/h (Kilograms per Hour): Often used in European industrial or diesel applications.
function calculateMassFlow() {
// 1. Get Inputs
var dispLiters = parseFloat(document.getElementById("engineDisp").value);
var rpm = parseFloat(document.getElementById("engineRPM").value);
var vePercent = parseFloat(document.getElementById("volEff").value);
var cycle = parseInt(document.getElementById("engineCycle").value);
var pressureBar = parseFloat(document.getElementById("intakePressure").value);
var tempC = parseFloat(document.getElementById("intakeTemp").value);
// 2. Validate Inputs
if (isNaN(dispLiters) || isNaN(rpm) || isNaN(vePercent) || isNaN(pressureBar) || isNaN(tempC)) {
alert("Please enter valid numeric values for all fields.");
return;
}
if (dispLiters <= 0 || rpm < 0 || vePercent < 0 || pressureBar * 60)
var massFlowLbMin = massFlowKgS * 2.20462 * 60;
// kg/s to kg/h
var massFlowKgH = massFlowKgS * 3600;
// Volume Flow to CFM (Cubic Feet per Minute)
// 1 m^3 = 35.3147 ft^3
var volumeFlowCFM = actualVolFlow * 35.3147 * 60;
// 8. Display Results
document.getElementById("resGS").innerHTML = massFlowGS.toFixed(2) + " g/s";
document.getElementById("resLbMin").innerHTML = massFlowLbMin.toFixed(2) + " lb/min";
document.getElementById("resKgH").innerHTML = massFlowKgH.toFixed(2) + " kg/h";
document.getElementById("resCFM").innerHTML = volumeFlowCFM.toFixed(1) + " CFM";
document.getElementById("resDensity").innerHTML = airDensity.toFixed(3) + " kg/m³";
// Show result box
document.getElementById("resultBox").style.display = "block";
}