Cold Start, Warm-up, Catalyst Heating: What the Software Controls

You start the engine in the morning, and for a minute or two it sounds different from later on the motorway: idle speed is raised, the sound a little rougher, and on a diesel the glow plug light comes on briefly first. That is neither coincidence nor a defect, but a finely calibrated programme in the engine ECU. The first minutes after a cold start are the most critical phase in an engine's life for wear and exhaust emissions. In this article we explain what the software controls during this time, why it does so and what role this plays in tuning.

Why the cold start is so delicate

A cold engine has three problems at once. First, fuel vaporises poorly: part of it condenses on cold combustion chamber walls and in the intake instead of burning cleanly. Second, the oil is viscous and needs a few seconds to reach everywhere. Third, and decisive for the emission standard, the catalytic converter is still cold. A cat only works above a certain operating temperature, the so-called light-off temperature, which is several hundred degrees. Until then, exhaust gas passes through largely untreated.

A large share of the pollutants a modern petrol engine emits on a typical test drive is therefore produced in the first minutes after the start. Manufacturers tackle exactly this: with software.

Petrol: catalyst heating via ignition timing and idle

The most important measure on a petrol engine is active catalyst heating. To do this, the ECU retards ignition timing significantly. Combustion starts later, and a larger share of the energy goes into the exhaust as heat rather than into the crankshaft. So that the engine still runs smoothly, idle speed is raised and cylinder filling increased at the same time. More exhaust mass at higher temperature brings the catalytic converter up to operating temperature quickly.

Further building blocks:

• Cold-start enrichment: because part of the fuel condenses on cold walls, the ECU injects more fuel at the start and in the first seconds. The amount depends on coolant and intake air temperature and is reduced quickly.
• Lambda control: the lambda sensor is electrically heated so that it measures as early as possible. Until it is ready, the ECU works with pre-controlled values, then in closed loop.
• Secondary air: some engines use a secondary air pump to blow additional fresh air behind the exhaust valves. The oxygen reacts with unburnt components in the hot exhaust and heats the cat further.
• Injection strategy on direct injection engines: multiple injections and adapted injection timing improve mixture formation in a cold combustion chamber and reduce particulates.

Diesel: glow plugs, idle and heating strategy

On a diesel the task is similar, the means are different. The glow plugs make sure before the start that the compressed air gets hot enough for auto-ignition. Modern systems keep glowing after the start. This post-glow stabilises combustion, reduces noise and white smoke and lowers emissions during warm-up.

In addition, the ECU controls:

• Raised idle for stable running and faster warm-up.
• Adapted injection: pilot injections and shifted injection timing make combustion in a cold engine softer.
• EGR during warm-up: how much exhaust is recirculated depends strongly on temperature, because recirculation can destabilise combustion in a cold engine.
• Heating strategy for exhaust aftertreatment: oxidation cat, particulate filter and SCR system need temperature too. Targeted injection strategies and throttling of intake air bring them to operating temperature faster. For the SCR system this is a prerequisite for AdBlue dosing to start at all.

Engine protection during warm-up

Besides emissions, it is about the engine itself. Many ECUs limit available torque or maximum engine speed while the oil is cold. Electronically controlled thermostats keep the cooling circuit closed at first so that the engine warms up faster. On a turbo engine this protects the turbocharger, whose bearings depend on free-flowing oil. This logic is why you should not put full load on a cold engine straight away, even if it would technically allow it.

What this means for tuning

A sound performance calibration changes the full-load and part-load areas where power is produced. The cold-start and warm-up strategy is not part of that. In a clean file it remains as the manufacturer designed it, including the temperature-dependent torque limit with cold oil. That is also in the interest of durability: especially on a tuned engine, a gentle warm-up matters.

Alongside this, a function called "Cold Start Delete" circulates in the tuning scene. It means switching off the catalyst heating strategy: no raised idle and no retarded ignition after a cold start. Technically, this is possible via maps on many petrol ECUs. The result is that the cat reaches temperature later and cold-start emissions rise. That makes the function emission-relevant: it interferes with the emission strategy that is part of the type approval. If secondary air is also disabled or its monitoring suppressed, it is emissions tampering, which is not permitted on public roads.

For all software changes, in addition: the operating permit expires, and the functions are intended exclusively for motorsport. Details in the motorsport notice.

Phase	Petrol	Diesel
Start	Cold-start enrichment	Pre-glow
First minutes	Retarded ignition, raised idle, secondary air where fitted	Post-glow, raised idle, adapted injection
Warm-up	Lambda control active, cat at temperature	Heating strategy for cat, DPF and SCR, temperature-dependent EGR
Engine protection	Torque limit with cold oil	Torque limit with cold oil

What you can do yourself

The software does a lot, but not everything. A few simple rules help the engine and the exhaust aftertreatment:

• Drive off instead of idling to warm up: under light load the engine warms up faster than at idle.
• Low revs and moderate load until oil temperature (not just coolant temperature) is in the normal range.
• Avoid lots of short trips, or plan regular longer drives, so that the cat, particulate filter and oil circuit warm through properly.

Conclusion

The first minutes after a cold start are a well-thought-out programme: retarded ignition and raised idle heat the cat, enrichment and injection strategy stabilise combustion, and on a diesel glow plugs and a heating strategy for exhaust aftertreatment are added. This logic is part of the emission strategy and engine protection. A clean tuning calibration leaves it untouched. The software options available for your ECU are shown in the configurator.