Mercedes-Benz Premiers New Gasoline Direct Injection System for More Power and Lower Fuel Consumption
|The direct-injection engine with aftertreatment system. Click to enlarge.
Mercedes-Benz has introduced the world’s first gasoline engine with piezoelectric direct injection and spray-guided combustion—the Stratified-Charged Gasoline Injection (CGI) engine. The spray-guided direct injection system first appeared in a mild-hybrid concept car Mercedes-Benz showed at the Frankfurt show in 2005. (Earlier post.)
The new 215 kW (292 hp) 3.5-liter six-cylinder engine will enter the market in the second half of 2006 in the CLS-Class as the CLS 350 CGI. In the European combined driving cycle, the gasoline direct injection system improves fuel consumption by 10% over the counterpart V6 gasoline engine with port injection and fully variable valve timing.
Estimated fuel consumption for the CLS 350 CGI is 9.1 liters/100km, or 26 mpg US.
The spray-guided injection achieves better fuel efficiency, and thus also higher thermodynamic efficiency, than conventional wall-guided direct injection systems. The new system will form the basis for future engine development work in this output class.
The main advantage of the CGI engine is in the stratified operating mode from which it takes its name. During this mode the engine is run with high excess air and thus excellent fuel efficiency.
Multiple injection extends this lean-burn operating mode to higher rpm and load ranges too. During each compression stroke, a series of injections takes place, spaced just fractions of a second apart. This improves mixture formation, combustion and fuel consumption.
While stratified charge operation was previously only possible in the low part-load range, the new Mercedes direct-injection engine can still operate in this lean-burn stratified mode at speeds in excess of 120 km/h (75 mph). Above this, the engine switches to homogeneous operation where the fuel/air ratio is 1:14.6 (lambda = 1).
When driving on main roads and highways at largely constant speed and with proper anticipation, the CGI engine outperforms the fuel economy of the six-cylinder engine with conventional injection technology by up to 1.5 liters per 100 km, a saving of up to 15%.
The engine also delivers 15 kW (20 hp) more power than the conventional-injection V6 and 4% more torque (365 Nm).
|The CGI injection system. Click to enlarge.
Injection system. The fast-acting, high-precision piezoelectric injectors are the critical enablers to the system. The piezoelectric valves have injectors which open outwards to create an annular gap just a few microns wide. This gap shapes the fuel jet and produces a uniform, stable, hollow-cone-shaped spray pattern.
The mixture formation itself is also enhanced by turbulences at the edges and inside the cone-shaped spray; these suck air particles into the fuel spray, forming an optimally ignitable mixture.
The microsecond response times of the piezoelectric injectors provide the basis for delivering multiple injections per compression stroke, and thus for lean-burn operation. By allowing flexible and efficient control of the combustion process they play a key part in ensuring the engine’s improved fuel efficiency.
With the aid of simulations for the fuel mixture and the combustion process, the pistons have been designed with special piston bowl geometry which concentrates the lean mixture in the area around the spark plug and prevents it from spreading out towards the cylinder wall. The piston shape therefore also plays its part in ensuring near-total combustion, low fuel consumption and low emissions in the direct-injection petrol engine.
A high-pressure pump and downstream fuel rail and pressure control valve are responsible for delivering the fuel and regulating the quantity supplied. The peak fuel pressure in this system is up to 200 bar—around 50 times the fuel pressure in a conventional gasoline injection system.
The pump delivers fuel to the rails during every second injection, building up maximum pressure. As fuel is only delivered on every second injection the pressure is slightly reduced during the cycle, however the mean pressure for all injectors remains at 200 bar during injection.
A regulating valve ensures that only the fuel quantity required for the engine’s operating point is delivered, thereby reducing the power requirement of the high-pressure pump.
Fuel that is not needed flows back via a water heat exchanger and is mixed with the incoming fuel from the tank of the CLS 350 CGI. The low-temperature coolant circuit of the injection system also cools the electronic control unit of the direct-injection engine, which manages all the working processes of this six-cylinder power unit.
Spark plugs. Correct positioning of the spark plugs was a challenge requiring sophisticated flow calculations and tests. To ensure that the ignition spark is able to jump rapidly and reliably, the spark plug must reach the cloud of fuel/air mixture but must not be in direct contact with the liquid fuel, otherwise it will gradually carbonize.
In order to meet both requirements, the piezo-injector of the CGI engine extends into the centre of the combustion chamber. It has therefore been moved roughly to the position where the spark plug is located in a conventional port-injection engine; the spark plug has been repositioned closer to the exhaust valves, where it can reach the ignitable mixture at the turbulent edges of the cone-shaped spray. A cross-flow cooling system in the cylinder head ensures that the spark plugs and injectors always operate in the most favorable temperature range.
|The aftertreatment system. Click to enlarge.
Emissions. The emission control strategy of the new CGI engine is based both on in-engine measures to deliver low engine-out emissions and on effective exhaust gas aftertreatment by a total of four catalytic converters.
The in-engine measures include the Mercedes-developed combustion process featuring multiple closely spaced injections on each compression stroke. This improves the exhaust quality of the V6 engine in the warm-up phase, as actively controlled injection and combustion using low quantities of fuel ensures higher temperatures in the exhaust manifold and accelerates catalytic converter warm-up.
Measurements show that engine-out hydrocarbon emissions in the warm-up phase are almost halved. Furthermore, since the injection and combustion processes can be actively controlled, it is also possible to raise temperatures in the exhaust manifold and thus speed catalytic converter warm-up. Just ten seconds after starting from cold, the direct-injection petrol engine reaches an exhaust temperature of over 700º C.
Aftertreatment begins with two close-coupled three-way catalytic converters, each of them monitored by two oxygen sensors—a control sensor and a diagnostic sensor. This linear oxygen sensor control goes into operation immediately after the engine starts from cold, providing information about the exhaust gas constituents which the electronic control unit of the V6 uses for a controlled warm-up.
To reduce nitrogen oxide emissions, Mercedes-Benz first uses dual electrically controlled and cooled exhaust gas recirculation (EGR) which, depending on engine operating conditions, redirects up to 40% of the exhaust gases back into the cylinders.
Secondly, it also uses underfloor NOx storage-type catalytic converters. Under lean operating conditions, these converters adsorb the oxides of nitrogen. Periodically, during brief regeneration pulses, the nitrogen oxides are then desorbed, reacting with other exhaust gas constituents to form harmless nitrogen. The NOx storage-type catalytic converters are also monitored by sensors—a temperature and a nitrogen oxide sensor.
As conventional catalytic converters require a stoichiometric fuel-air mixture (lambda = 1), but stratified charge operation uses high excess air (lambda >1), the CLS 350 CGI is equipped with two NOx storage-type catalytic converters.
Other engine technologies. The CGI is based on the port-injected V6 first introduced in 2004. Other engine technologies featured on the engine include:
Variable camshaft timing on the intake and exhaust sides improves the available output. The camshaft angles are adjustable by anything up to 40 degrees to ensure that the valves are able to open and close at the most favorable time in any driving situation.
A variable intake module varies the air supply as required. The length of the intake ducts leading to the cylinders is adjusted by means of flaps: at lower engine speeds the flaps are closed to increase the length of the intake duct. This creates pressure waves which support the intake process and make a lasting improvement to the torque yield in the lower engine speed range. As a result 317 Newton metres—around 87 of the maximum torque—is already available from 1500 rpm.
Fuel economy is improved by an intelligent thermal management system. Coolant circulation is stopped during the warm-up phase, so that the engine reaches its normal operating temperature more rapidly. The result is an improved oil flow and considerably less in-engine friction, as well as lower exhaust emissions. When the warm engine is operating under full load, the thermal management system always keeps the engine oil and coolant at the best possible temperatures. This ensured by an electronically controlled thermostat which is active in all driving situations.
Aluminum cylinder head and crankcase.
The cylinder liners are surfaced with a low-friction aluminium-silicon alloy. which has proved its worth in other Mercedes-Benz car engines. Other advantages include high dimensional stability, exemplary thermal characteristics and low weight. The weight-saving compared with conventional grey cast-iron liners is around 500 grams per cylinder.
The forged crankshaft is equipped with four counterweights. Four wide crankshaft bearings with transverse reinforcing struts attached to the crankcase also help to reduce vibrations. A balancer shaft between the two cylinder banks compensates the free vibration moments which are inherent to a V6 engine, ensuring exemplary smooth running. It counter-rotates at the same speed as the crankshaft.
The new CLS 350 CGI is designed to operate on sulfur-free unleaded premium fuel. In Western Europe, the CLS direct petrol injection model will replace the current CLS 350.