Exhaust temperature, richness and air conditioning — a link?

1. What is the exhaust gas temperature?

Exhaust gas temperature (EGT) is a crucial indicator in thermal engines.
It depends on many parameters: air/fuel mixture richness, ignition timing, boost pressure and combustion efficiency.

Naturally aspirated petrol engine: 600 to 900 °C
Turbo petrol engine: up to 950–1050 °C

An excessive temperature can cause knock and damage valves, manifolds, turbo.

2. ⚖️ The role of the air/fuel richness

The mixture richness is expressed via the stoichiometric ratio (λ = 1 → 14.7:1 for petrol).

Lean mixture (λ > 1): more air → hotter combustion
Rich mixture (λ < 1): more fuel → cooler combustion

Why does a rich mixture cool?

The excess fuel is not completely burned. As it vaporises, it absorbs a large amount of thermal energy via the latent heat of vaporisation (~350 kJ/kg for petrol).

This reduces:

The temperature in the combustion chamber,
The exhaust gas temperature,
And protects the hot components (turbo, valves, manifolds).

Real example: 0.17 kg/h of petrol → Q = m × Lv = 0.17 × 350 = 59.5 kJ/h ≈ 16.5 W dissipated continuously.
Warning: A mixture that is too rich is harmful, mixed lubrication (oil/petrol mixture) → degradation of the cylinders

3. ❄️ The role of the air conditioning evaporator

The link between temperature and air conditioning lies in the thermodynamic operation of the evaporator, an essential component of the refrigeration circuit.

When the air conditioning is activated:

The refrigerant (R134a or R1234yf) enters the evaporator in liquid form at low pressure,
It vaporises by absorbing the heat of the ambient air passing through the evaporator,
This change of state (liquid → gas) absorbs a large amount of heat (latent heat of vaporisation),
This cools the air blown into the cabin.

The principle is identical to that of the fuel in the combustion chamber: a fluid turns into a gas by absorbing the heat of its environment.

Comparison: petrol vs refrigerant

Phenomenon	Fluid	Effect
Combustion chamber cooling	Petrol	Vaporisation → heat absorption → temperature drop
Cabin cooling	R134a / R1234	Vaporisation → heat absorption → temperature drop

4. ⚠️ Why it matters?

In competition, the mixture is deliberately enriched under load to avoid failure (turbo, valves…)
On a standard engine, thermal management is automated and mechanically protects
On a modified or remapped engine, poor richness management can lead to overheating

5. In summary

Factor	Effect on exhaust T°
Lean mixture	? Increases
Rich mixture	❄️ Decreases

Key takeaway

Richness acts as an internal thermal regulator.
The air conditioning evaporator also works on the principle of latent heat of vaporisation.
These two systems share a common physical logic: lowering the temperature via a change of state.

To go further

To explore further the link between the air/fuel ratio (lambda) and exhaust gas temperature, as well as the role of injectors in this phenomenon, see these two references:

SAE Technical Paper 2001‑01‑1339 – Calculated A/F Ratio via Exhaust Gas Temperature Measurement for Small Engine Control
This publication demonstrates that measuring the gas temperature allows the air/fuel ratio to be estimated, with a maximum temperature at lambda ≈ 1 and drops at rich or lean extremes.
Practical article – Inspection and cleaning of petrol injectors
Your article on mespiecesauto.com clearly presents how injectors work, their influence on combustion and temperature, and the flow-testing and cleaning techniques.
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Exhaust temperature, richness and air conditioning?