function calculateExhaustFlow() {
// Get Inputs
var disp = parseFloat(document.getElementById('engineDisp').value);
var rpm = parseFloat(document.getElementById('engineRPM').value);
var ve = parseFloat(document.getElementById('volEff').value);
var cycle = parseInt(document.getElementById('engineCycle').value);
var tIn = parseFloat(document.getElementById('intakeTemp').value);
var tExh = parseFloat(document.getElementById('exhTemp').value);
// Validation
if (isNaN(disp) || isNaN(rpm) || isNaN(ve) || isNaN(tIn) || isNaN(tExh) || disp <= 0 || rpm V2 = V1 * (T2/T1)
// Temperatures must be in Kelvin
var tempInK = tIn + 273.15;
var tempExhK = tExh + 273.15;
var expansionRatio = tempExhK / tempInK;
// Exhaust Flow in m3/min
var exhaustFlowM3Min = intakeFlowM3Min * expansionRatio;
// 3. Conversions
// CFM = m3/min * 35.3147
var flowCFM = exhaustFlowM3Min * 35.3147;
// m3/h = m3/min * 60
var flowM3H = exhaustFlowM3Min * 60;
// L/s = (m3/min * 1000) / 60
var flowLS = (exhaustFlowM3Min * 1000) / 60;
// Mass Flow Calculation (Based on Intake Air Mass)
// Mass = Volume (Intake) * Density (Intake)
// Density varies by temp, but roughly:
// Density at T_in = 1.292 * (273.15 / T_in_K)
var densityIn = 1.292 * (273.15 / tempInK); // kg/m3
var massFlowKgMin = intakeFlowM3Min * densityIn;
// Add fuel mass (approx 1/14.7 for stoich, roughly +6-7%)
// Let's assume AFR 12.5:1 for power, so +8% mass
var totalMassFlowKgH = (massFlowKgMin * 1.08) * 60;
// Display Results
document.getElementById('resCFM').innerText = flowCFM.toFixed(1) + " CFM";
document.getElementById('resM3H').innerText = flowM3H.toFixed(1) + " m³/h";
document.getElementById('resLS').innerText = flowLS.toFixed(1) + " L/s";
document.getElementById('resMass').innerText = totalMassFlowKgH.toFixed(1) + " kg/h";
document.getElementById('egfResultBox').style.display = 'block';
}
Understanding Exhaust Gas Flow Rate
Calculating the exhaust gas flow rate is a critical step in automotive engineering and performance tuning. It determines the necessary specifications for exhaust piping diameters, catalytic converters, mufflers, and turbocharger turbine housings. If an exhaust system is too small for the calculated flow rate, it creates excessive backpressure, robbing the engine of power and potentially causing overheating. Conversely, an exhaust pipe that is too large may reduce the scavenging effect in naturally aspirated engines, reducing low-end torque.
How the Calculation Works
The exhaust flow rate is primarily derived from the volume of air the engine pumps in, corrected for the thermal expansion that occurs during combustion. Since an engine is essentially an air pump, we start by calculating the intake volume and then apply physics laws to determine the exhaust volume.
The core formula involves two main steps:
1. Intake Volume Calculation: We calculate how much air enters the engine based on Displacement, RPM, and Volumetric Efficiency.
2. Thermal Expansion: According to Charles's Law, the volume of a gas is directly proportional to its temperature (in Kelvin). Since exhaust gas is significantly hotter than intake air, it occupies much more space.
Key Variables Explained
Variable
Description
Typical Values
Displacement
The total volume swept by the pistons in the engine.
1.6L – 6.2L (Passenger Cars)
Volumetric Efficiency (VE)
How effectively the cylinders are filled with air compared to their static volume.
80-90% (Stock), >100% (Turbo)
Exhaust Temperature (EGT)
The temperature of the gas leaving the combustion chamber. Higher temps mean higher volume flow.
700°C – 900°C (Gasoline)
Why is Exhaust Flow Higher than Intake Flow?
You might notice the calculated exhaust CFM is significantly higher than the intake CFM. This is due to Thermal Expansion. When air and fuel combust, the chemical reaction releases heat, raising the temperature of the gas from roughly 25°C (intake) to 800°C+ (exhaust).
While the mass of the gas changes slightly due to the addition of fuel, the volume changes drastically. For example, a gas heated from 300K (27°C) to 1100K (827°C) will expand to nearly 3.7 times its original volume. This is why a 3-inch exhaust pipe is necessary for a high-horsepower engine that might only have a 3-inch intake pipe; the hot gas simply needs more room to move efficiently.
Example Calculation
Let's look at a realistic scenario for a modified sports car: