Internal Combustion Engine
The internal combustion engine in the Twin Engine is available in two variants depending on market. The USA market uses the B4204T28 engine, which has the emission rating TZEV (Transitional Zero Emission Vehicle). For the EU and the rest of the world, the engine is designated B4204T35 and is adapted for the Euro 6 emission rating.
The engines are based on the B4204T20 engine.
Exhaust after-treatment
Catalytic converter
The catalytic converter has a compact design and is fitted close to the engine to provide faster heating, good flow and effective cleaning. There are differences between the catalytic converters for the different markets.
The USA market's TZEV emission rating has slightly tougher emission requirements than Euro 6, and therefore needs a larger front substrate with a different composition of precious metals in the coating. The larger substrate means higher back-pressure through the catalytic converter. For this reason, the B4204T28 engine (USA) has a power output that is six horsepower lower than the B4204T35 (EU).
Catalytic converter heating
During a cold start of the internal combustion engine, CISG is used as load in order to obtain faster heating. Heating takes place in two phases. The time for each phase differs depending on market.
Phase 1 continues for approx. 15 seconds (EU) or approx. 35 seconds (USA). The engine runs at increased idle with as small a load as possible. This is the only situation where the internal combustion engine can run without being responsible for the car's propulsion.
Phase 2 continues for approx. 35 seconds (EU) or approx. 50 seconds (US). The catalytic converter is heated to some degree, and the engine is loaded by CISG equivalent to maximum battery charging.
The engine is never switched off during the catalytic converter's heating phase in order that the hybrid operation does not change over to electric operation, for example, but the process always finishes if it has been started.
Lambda probes
Depending on the engine's emission rating, the engine can have two or three lambda probes that measure the oxygen content in the catalytic converter.
- The B4204T28 engine (USA) has three lambda probes.
- The B4204T35 engine (EU) has two lambda probes.
Lambda probes, B4204T28 engine (USA)
The front of the three lambda probes is used to control the fuel-air mixture to the mixture ratio 14.7:1 (lambda 1).
Under certain operating conditions, e.g. during the heating phase directly after starting, other mixture ratios are required in order to achieve rapid catalytic converter activation and thereby low emissions. The center and the rear lambda probes are used for this.
The center lambda probe records the oxygen content in the center of the front catalytic converter. Since the center section of the catalytic converter heats up more quickly than the rear, the function of the catalytic converter can be controlled more quickly. During a cold start (outside temperature 25 °C) it takes approx. 15 seconds for the front section of the front substrate to reach a sufficiently high temperature for catalytic converter regulation to be initialized.
After approx. 70 seconds, the whole of the front substrate is warmed up, and the rear lambda probe is activated. The rear lambda probe is used during the remainder of the driving cycle to regulate the fuel-air mixture and to diagnose the front catalytic converter monolith.
During certain time periods, the fuel-air mixture is either leaner or richer than lambda 1. To be able to oxidize carbon monoxide and hydrocarbons, the engine is run lean during certain time periods in order to store oxygen in the catalytic converter. At other times, the engine is run rich in order to reduce stored nitrogen oxides.
Lambda probes, B4204T35 engine (EU, rest of the world)
The front of the two lambda probes is used to control the fuel-air mixture to the mixture ratio 14.7:1 (lambda 1).
The position of the rear lambda probe is equivalent to the center lambda probe in engine variants with three lambda probes. The rear lambda probe records the oxygen content in the center section of the catalytic converter. The position means that catalytic converter regulation can be initialized approx. 20 seconds after starting, when the catalytic converter monolith's front half has reached operating temperature. The lambda probe's signal is then used during the whole driving cycle for catalytic converter regulation and for diagnosis of the catalytic converter function.