Evolution des Vierzylinder-Dieselmotors von Mercedes-Benz

You will find the figures mentioned in this article in the Germ an issue of MTZ 01I2007 beginning on page 20.

Evolution of Mercedes-Benz’s

Four-Cylinder Diesel Engine

Authors:Joachim Schommers, Franz-Paul Gulde, Martin Hoppenstedt, Gerhard Gruber, Wolfgang Fick, Wilhelm Ruisinger, Gregor Renner and Andreas Lingens

For years now, Mercedes-Benz’s pro-ven four-cylinder diesel has been in demand as an entry-level power solu-tion, particularly for the mid-range sedans of the C- and E-Classes. In pa-ce with refinement of the E-Class and in preparation for the new C-Class, this tried and true power unit has re-ceived another significant upgrade of its power output and torque. A whole range of measures has been taken to bring its longevity and ease of hand-ling to still higher levels. The result is a remarkable product that meets cus-tomers’ demands for road behaviour, silence and good fuel economy.

1 Introduction

Point of departure for development was the engine that DaimlerChrysler knows as the “OM646”, introduced in 2002 in the E 200 CDI and E 220 CDI models of the cur-rent E-Class, and today powering the C-Class C 200 CDI and C 220 CDI as well. With more power and torque, this engine will be an even more attractive option, while longevity and driving also have to be increased. To this end, and to compen-sate for the higher loadings to which com-ponents are subjected, a series of design refinements was called for, leading to an evolutionary development of the engine. In June 2006, the engine, under the desig-nation OM 646 EVO, was brought to mar-ket as the base diesel engine in the fresh-ened E-Class.

2 Objectives and Strategies

2.1 GoalsThe development goals were:– 14 % increase in maximum engine out-

put (from 110 to 125 kW)– maximum torque raised by 18 % (340

Nm to 400 Nm)– adoption of an optimized injection sys-

tem– improvement over the already high NVH

qualities of its predecessor– faster glow-plug pre-heating– compliance with Euro 4 legislation both

with and without particle filter– optimization / standardization of com-

ponents and assembly processes– reduced fuel consumption.To achieve these goals, development was focused on the following areas:

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– upgrading of engine internals to han-dle tougher mechanical and thermal stresses

– development of innovative control and regulation strategies with a new injec-tion system

– engine NVH optimization – adoption of a ceramic pre-heating sys-

tem.

2.2 Achieving Increased Horsepower and TorqueThe follow measures contribute to the in-crease in power output:– lowering compression ratio from 18 to

17.5 – raising peak pressure by 10 bar– optimizing the charge-air path and re-

duction of gas cycle losses by modifying swirl and flow

– use of an exhaust turbocharger with op-timized efficiency and control kinemat-ics.

– more efficient charge air cooling– elevation of maximum exhaust gas tem-

perature from 780 °C to 815 °C. – limitation by means of temperature sen-

sor and associated control strategy.The Table shows comparative data for evolved and original engines.

3 Reworking of Engine and Cylinder Head

To handle higher peak pressure loads, cer-tain changes were made to the engine.

3.1 PistonsPiston compression height was raised by 2 mm. This resulted in a new piston mould-ing having a two-part cooling channel, Fig-

ure 1. The conductive surfaces are enlarged by giving the cooling channel a waveform shape.

3.2 Connecting RodWhen compression height was raised by 2 mm, the piston rods were shortened by 2 mm so as not to alter the main dimensions of the engine.

Use of a higher-strength material made meant that the geometry of the rod could be optimized for weight and function. Thus, piston rod weight could be reduced by 52 g, thereby compensating for the heav-ier piston.

A moulded bore in the piston rod little end obviates edge pressure. Rod bearings are sputter bearings with high-strength steel backing and finely distributed bronze, Figure 2.

3.3 Cylinder HeadTo deal with higher thermal impacts, meas-ures were taken to optimize cylinder-head cooling. These include:– changes to the water jacket for improved

high-cycle-fatigue endurance– optimized cooling of the combustion

chamber plate by means of a partial in-termediate deck in the water jacket.

– de-constricting coolant flow by means of combustion chamber plate transfer openings optimized for inflow

– increased coolant through-flow in the web coolant passages by reducing over-flow losses from the cylinder-crankcase to cylinder head at the cylinder-head gasket water passages

– inlet-side V-shaped cooling channels in the valve reinforcement ribs.

Figure 3 shows comparative views of the OM 646 and OM 646 EVO water jackets. More particularly, we can see the results of the V-shaped channels: it is evident that com-bustion chamber cooling has been signifi-cantly improved over that of the predeces-sor model.

For a stronger cylinder-head gasket seal, higher-quality cylinder-head bolt material has been adopted and bolt force increased by 9 %. The screw bosses on the crankcase have been reinforced to accept the higher cylinder-head bolt forces.

4 Gas Exchange

As compared with the predecessor model, overall gas circulation has been further improved. Air, charge air and exhaust gas recirculation have all been considered, Figure 4.The main focus has been on:– minimizing flow losses– optimizing charge air cooling– minimizing air passage noise– a modular concept for EGR cooling and

EGR cooler bypass– exhaust-driven turbocharger with opti-

mized efficiency and vane adjustment kinematics.

4.1 Charge-Air Distribution LineFor the OM 646 EVO, a very compact design solution was achieved by shortening the charge and swirl duct in the charge-air dis-tribution line to 65 mm.

Advantages are the creation of installa-tion space for higher-performing EGR cool-er, partially including integrated bypass flap, lower noise emission resulting from reduced surfaces particularly of plastic

components, improved charge-air build-up, faster transition from EGR to full-load op-eration, since little exhaust gas is passed forward.

These last measures significantly in-crease agility.

4.2 Air IntakeAdditional volume between air filter and turbocharger made it possible to stimulate air admission conditions in the compressor reservoir in such a way that the charge pressure in the pump line, which limits available torque, could be raised by about 200 mbar. In this way, the engine torque curve is higher below 1800 rpm, with clear advantages in both stationary and non-sta-tionary operation.

The component consists of two plastic half-shells joined by vibration welding.

4.3 TurbochargingThe basis for a new development of a VNT exhaust turbocharger specific to this project was Honeywell’s new “step 3” turbocharger. In this unit, the operational reliability of the adjusting mechanism has still been im-proved with respect to the increased load.

This is possible because thermal elonga-tion can occur without deformation. At the same time, a nickel-free material is now used for the turbine housing.Other details have been improved:– electrical vane pitch control optimized

for torque– better turbine performance from guide

vanes optimized for hysteresis and effi-ciency

– improved sealing between turbine and bearing housing owing to high exhaust-gas back-pressure

– gas-nitrided insert for minimal friction between pitch control vane and insert.

To ensure that the turbocharger operates in the permissible temperature range, an exhaust gas temperature sensor is located in the turbine housing. This enables ex-haust gas temperature to be safely kept within limits in all operating phases and throughout the running time.

4.4 Charge Air ControlTo adjust to higher temperatures down-stream of the compressor and the increased charge pressure, the charge air tube (down-stream of the turbocharger) has been switched to a material with greater ther-mal resistance.

The charge air cooler fitted to the vehi-cle is identical with that of the OM 642 V-6 engine, which affords the required increase in charge-air density.

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For reasons of installation space, the air coming from the charge air cooler is direct-ed by the engine mount into the intake air course.

In the “Euro 4 with diesel particle fil-ter” version, intake air throttling (ALD) takes place at the engine mount, taken over from the gasoline and diesel engines components.

For the “Euro 4 without diesel particle filter” emissions level, the ALD is replaced by a plastic intermediate flange.

The exhaust, mixed with charge air, is led into the charge-air distributor line via the mixing chamber with charge pressure sensor and charge air manifold.

The charge-air distributor line contains a charge and swirl port 65 mm long for each cylinder. Balanced EGR distribution has been achieved through simulation and tests. For swirl control, the charge port can be steplessly closed by an electrically-actu-ated throttle flap.

4.5 Exhaust Gas RecirculationThe exhaust gas, pre-cooled by the cylinder head and a water-cooled EGR-line, arrives on the cold side of the engine. It is of modu-lar design and contains the electrical EGR valve – part of an internally-produced as-sembly, Figure 5.

Depending on the application, either a pneumatically-actuated EGR bypass flap or a closing flange is installed. Both valves are designed as plug-in units. Coolant water is led above and in parallel with the exhaust gas in the EGR line and thereby reduces not only the temperature of the exhaust gas but that of the integrated components as well.

The newly introduced EGR bypass flap, in two switching positions, guides the ex-haust gas either through the EGR cooler or directly into the mixing chamber through the EGR duct.

4.6 Exhaust Gas Recirculation CoolingThe intake and discharge ports of the EGR cooler are accommodated on one and the same flange face – the exhaust gas thus performs a 180° reversal of direction in the cooler.

Depending on the particular case – ‘’with” or “without” diesel particle filter – different exhaust gas recirculation coolers are used:– with DPF: EGR cooler in sand-cast alu-

minium with a gas channel through which ribs run

– without DPF: stainless steel cooler with two times 26 swirl tubes, arranged par-allel to each other.

Coolant is taken from the crankcase, flows through the EGR cooler, and then to the oil/water heat exchanger.

5 The Common Rail Injection System

As part of the engine’s evolutionary proc-ess, a Delphi solenoid-valve controlled in-jection system has been adopted. New features as compared to the previous system are:– use of an injector with 17 mm shaft di-

ameter– use of a venturi nozzle in the low-pres-

sure circuit to set up a defined vacuum in the injector return flow

– induction-side fuel quantity regulation of high-pressure pump for high-pressure generation in response to needs

– compensation for fuel quantity drift by means of body vibration sensor control.

5.1 InjectorThe injector is a very compact and simple design with a pressure- compensated servo valve in the near of the nozzle, Figure 6. A new feature is that the solenoid is located in the shaft.

It is distinguished by the least possible dead space in the hydraulic circuit and sig-nificantly reduced leakage as compared to the predecessor system.

Some advantages of the injector are minimal shot/shot variations, good regula-tion of pilot fuel quantities at high rail pressures, steeper injection rates at low rail pressures (positive for emissions behaviour and the minimal demonstrable rail pres-sure in the idle range [NVH]).

As with the previous engine, a flow-opti-mized seven-hole nozzle is used.

5.2 Fuel Quantity Drift Compensation StrategyTwo procedures have been used to ensure optimum equalization of injection behav-iour both in new condition and over the life of the system.

5.2.1 Coding at Initial OperationInjector correction values are saved in a data matrix code on the injector body and transferred to the engine control unit at the assembly plant. These values were de-termined earlier in the production of the injector by recording the injector perform-ance map at various rail pressure stages.

In this way, an optimal equalization is obtained. It was possible to reduce signifi-cantly the influence of manufacturing variations on measurement accuracy.

5.2.2 Body Vibration Sensor Control in Operation

Changes in fuel injection quantity over the life cycle of the injector caused, for ex-ample, by wear of the nozzle metering nee-dle seating, are recognized and regulated by body vibration sensor control. The pro-cedure is illustrated in Figure 7. The body vibration sensor installed on the crankcase detects an oscillation signal in the 4-8 kHz range, caused by the combustion of a small pilot injection quantity. In what is known as a “learning mode”, the pilot injection quantity at each injector will be increased until this reference signal can be detected.

By determining the difference from the reference control time with the value actu-ally measured, deviations in the pilot injec-tion quantity can be corrected. And it is then possible to correct higher injection quantity ranges by extrapolation. Body vi-bration control operates throughout the entire performance map.

6 Glow Plug System

When the compression ratio is lowered, de-mands on the glow plug system increase.

To ensure easy starting, stable idling, and low exhaust and noise emissions, a rapid-start system with ceramic glow plugs has been adopted.

In comparison with traditional glow plugs, these plugs have a working tempera-ture about 200 °K higher with lower power consumption and longer service life.

Over wide temperature ranges, preheat-ing time has been reduced by a not imper-ceptible degree, Figure 8.

7 NVH Optimization

One task set in the requirements specifica-tion was to improve noise level and free-dom from vibration over the previous model.

Given the increased torque and power of the new engine, this posed a special chal-lenge.

Accordingly, the NVH behaviour of the complete engine was first optimized using finite-element calculation.

Natural vibrations analyses were per-formed on partial models for all the rele-vant components. This method identified potential noise sources early in the design phase, so that the necessary countermeas-ures could be taken. Significant improve-ments could be made by providing ribbing in the air ducting, for instance, or by deter-

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mining the correct attachment points for the engine cover.

Turning to the complete engine, sound emission behaviour was analyzed using partial sound source analyses and compo-nent structures were optimized. Optimized ribbing made possible a significant im-provement in the noise emission behaviour of the oil pan, for example.

Particular attention was paid to combus-tion noise, which stands in conflict with NVH and emissions requirements. This was optimized using a special technique, devel-oped by Mercedes-Benz, and based on a de-fined und level map over all engine speed and load ranges. It describes the permissi-ble combustion noise level, and how it changes with load and engine revolutions. The homogeneity of the map is an essential element in making subjective evaluations of noise behaviour.

Basic data used are indicated cylinder pressure curves. This technique enables the first noise evaluations to be made on the combustion test stands, which yields indi-cations for optimizing combustion devel-opment. The NVH measures taken not only either attained or surpassed combustion noise level target values over the entire per-formance map, but also achieved good ho-mogeneity.

Other measures to improve the subjec-tive noise impression included fitting the engine cover with an acoustically more ef-fective insulation and the adoption of a higher-density injector duct cover.

For freedom from vibration, all variants of the OM 646 EVO have a Lanchester har-monic balancer fitted.

Implementation of all these measures has brought noise levels in the relevant driving range as much as 2 dB below that of its predecessor, whether under full load as well as in deceleration, Figure 9.

In this way the exacting goal was at-tained of producing a smoother engine with a harmonic noise behavior.

8 Engine Data

8.1 Power Output and TorqueThe measures outlined in section 2.2 above yielded the power and torque curve depict-ed in Figure 10.

8.2 Emissions and Fuel ConsumptionCylinder-head charge behaviour has been improved, with flow being increased and swirl reduced by about 15 %. A stepless elec-trically adjustable inlet port shutoff (EKAS) is used to provide sufficient mixture forma-

tion energy under partial load. Nozzle con-figuration (injection aperture design, noz-zle projection, jet angle and position) has been optimized with the piston recess.

Together with the reduced compression ratio, the modular EGR cooling system that allows different cooling effects for the vari-ants and also a bypassing of the cooler, have all exerted a significant influence on the specifically low NOx emissions.

By optimizing the combustion process in this way, Euro 4 emissions standards were met for all power, transmission and vehicle variants, even without active ex-haust gas after-treatment.

Meanwhile, specific fuel consumption has been improved by up to 3 % over the predecessor engine. In the NEDC, that sig-nifies a fuel consumption lower by up to 0.2 l per 100 km, Figure 11.

8.3 PerformancePerformance has been raised to the level of the five-cylinder engine, now no longer built. At the maximum governed engine speed raised to 5000 rpm, the vehicle has clearly become more agile in passing ma-noeuvres and in acceleration through the gears. The motive power harmonizes beau-tifully with the sports-car type suspension and steering geometry. Performance differ-ences from the predecessor models in ac-celerating from 0 to 100 kph are also shown in Figure 11.

9 Summary

The OM 646 EVO provides an entry-level diesel for the C-Class and the refined E-Class that fully meets the expectations of customers of the Mercedes-Benz brand for power development, longevity, economy and comfort.