Combustion Properties
Specific Heat Ratio (k-factor)
The specific heat ratio (k, also written as 𝛾) is the ratio of a gas’s specific heat at constant pressure (Cp) to its specific heat at constant volume (Cv), written as k = Cp / Cv). For gas engines, this value dictates how much a fuel-air mixture heats up during compression and how fast fuel gas flows through metering valves.
Typical Specific Heat Ratios (k) for Engine Fuels
The molecular structure of a fuel gas determines its (k) value. Simple diatomic gases like hydrogen have higher values, while complex hydrocarbons or triatomic gases like carbon dioxide have lower values.
| Fuel / Gas Type | Primary Components | Typical k Value (at ~20°C / 68°F) | Impact on Gas Engine Operation |
|---|---|---|---|
| Hydrogen (H2) | 100% H2 | 1.41 | Causes higher compression temperatures and faster flame speeds. |
| Air (Baseline) | 78% N2, 21% O2 | 1.40 | Forms the bulk of the intake charge in lean-burn gas engines. |
| Natural Gas (Pipeline) | ~85–95% CH4 | 1.30 – 1.31 | Standard design baseline for most stationary gas engines. |
| Biogas / Landfill Gas | ~50–65% CH4, ~35–50% CO2 | 1.25 – 1.28 | High CO2 absorbs heat, lowering compression temperatures. |
| Propane / LPG | C3H8 / C4H10 | 1.13 – 1.15 | Low k value; requires careful knock control and timing adjustments. |
Why Specific Heat Ratio Matters in Gas Engines
Engine manufacturers and commissioning engineers track (k) across three critical areas:
- Compression Temperature and Knock. During the compression stroke, the temperature rise follows the relation T2 = T1 · (CR)k-1, where CR is the compression ratio. A higher k leads to higher end-of-compression temperatures, which increases the risk of engine knock (pre-ignition).
- Thermal Efficiency. In an ideal Otto cycle, thermal efficiency improves as k increases (η = 1 – 1 / CRk-1). Lean-burn engines run with high excess air (k ≈ 1.40) partly to keep the overall mixture k high and boost efficiency.
- Fuel Valve and Orifice Sizing. The speed of sound in a gas depends directly on k. Software like Caterpillar GERP or INNIO sizing tools uses k to calculate critical pressure drops and mass flow rates across gas admission valves and venturi mixers.