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A breakthrough in diesel engine technology, the common rail direct injection (CDI) system developed for the Vito van gives Mercedes-Benz a clear edge over its diesel-powered rivals in the light commercial vehicle market.
Among an array of benefits, the turbocharged and intercooled 2.2 litre CDI engine not only delivers exceptional torque output, but also delivers it at engine speeds as low as 1500 rpm. The result is an engine that is surprisingly strong at low revs and extremely fuel efficient in all conditions.
CDI also offers distinct environmental advantages with enhanced combustion efficiency leading to reduced exhaust emissions and quieter, smoother operation over comparable engines with either direct or indirect fuel injection.
Simply explained, common rail refers to the single fuel injection line on the CDI engine. Whereas conventional direct injection diesel engines must repeatedly generate fuel pressure for each injection, in CDI engines the pressure is built up independently of the injection sequence and remains permanently available in the fuel line.
In the CDI system developed jointly by Mercedes-Benz and Bosch, the electronic engine management system continually adjusts the peak fuel pressure according to engine speed and throttle position. Sensor data from the camshaft and crankshaft provide the foundation for the electronic control unit to adapt the injection pressure precisely to demand.
In other words, pressure generation and fuel injection are independent of each other, enabling fuel injection to be adapted to actual demand and consequently improving fuel efficiency and emission control.
Critical to this process is a special accumulator that can maintain injection pressure at a constant high level of up to 1350 bar (19,845 psi). This is the task actually performed by the common rail which is connected to all four injectors.
At the end of each injector, a rapid-acting solenoid valve adjusts both the injection timing and the amount of fuel injected. A microcomputer controls each valve's opening and closing sequence.
Unique intake and exhaust ports
The 2.2 litre CDI engine uses an aluminium cylinder head with two spiral intake ports, one for swirling the fuel/air mixture and the other for filling the combustion chamber.
Both ports are tuned to the symmetrically shaped combustion chambers and are designed to set the air into rapid swirling motion even before it reaches the cylinders. This ensures an optimal fuel/air mixture, especially in the part throttle range.
Inside the combustion chambers, newly developed injectors are positioned in the middle of the cylinder to promote uniform fuel distribution.
Another new feature of the CDI engine is the integration of a port in the cylinder head for the exhaust gas recirculation (EGR) system. In most diesel engines this system is routed around the outside of the engine but in the CDI system an EGR port has been cast into the cylinder head to channel gas from the exhaust side of the engine to the intake side.
This design has three distinct benefits: It dispenses with external EGR lines, transfers exhaust heat to the coolant for quicker engine warm-up, and at the same time cools exhaust gases to further enhance combustion.
Pilot Injection Feature
The high combustion pressure of up to 145 bar (2130 psi) in the Vito CDI engine, and the rate at which this pressure rises during the combustion process normally produce higher noise levels in direct injection engines than in their pre-chamber (indirect injection) counterparts.
However, the trail-blazing CDI system employs a piece of technical wizardry known as pilot injection' to overcome this problem: A few seconds before the main fuel injection, a small amount of diesel is injected into the cylinder and ignites, thereby establishing the combustion process and setting the ideal conditions for the main combustion process. Consequently, the fuel ignites faster with the result that the rise in pressure and temperature is less sudden.
The end effect, however, is not only a reduction in combustion noise but also a reduction in nitrogen oxide (NOx) emissions thanks to pilot injection and the lower cylinder temperature.
In a bid to lower noise even further, Mercedes-Benz engineers also developed a special cover for the cylinder head, and stiffeners on the oil pan, timing cover and crankcase.
The total effect of these measures is a CDI engine with noise levels below those of comparable pre-chamber engines.
Precise engine management
Like all modem Mercedes-Benz diesel engines, the Vito's CDI engine is governed by a high performance microcomputer which is connected by a CAN (Controller Area Network) database to the other control units on board the vehicle.
This electronic engine management network is a critical element of the common rail system because only the speed and spontaneity of electronics can ensure immediate pressure injection adjustment and cylinder-specific control of the injector solenoid valves.
"When it comes to technology and its associated benefits for operators and society alike, there is nothing on the Australian market to match the new Vito 108 CDI," said DaimlerChrylser AustraliWPacific's senior executive of Mercedes-Benz Trucks and Transporters, Mr Rod Baily.
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