DEVELOPMENT Engines You will find theDiesel figures mentioned in this article in the German issue of MTZ 7-8/2005 beginning on page 546. Die Entwicklung der Dieselmotorreihe i-CTDi von Honda

Development of the Diesel Engine Series i-CTDi by Honda Since the beginning of 2003, Honda is using its first own-developed diesel engine i-CTDi in the Accord. To respond to the strong market demand and the resolve of producing environmentally friendly cars, Honda will offer the well-established engine with a particulate filter as well. Models of the SUV-Segment already fulfil the Euro 4 standard even without a particulate filter, despite their bigger mass and their higher driving resistance.

By Kenichi Nagahiro, Tomoya Abe, Kenichi Okawara, Masakazu Yamazaki and Ikuro Hara

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1 Preliminary Remark

2 Development Concept

The interest in diesel engines with higher power and reduced CO2 emissions has increased over the last years. As a result, the share of diesel vehicles among overall passenger car sales in Western Europe is currently over 50 %. At the end of 2003, Honda also began production of common rail turbodiesel engines which it has fully developed by itself as a response to global warming and with the aim of enabling high-quality sporty passenger cars to be launched. Owing to its successful rate of market acceptance, this engine – which was initially only used in the Accord – has also been employed in the CR-V (model in the SUV segment) since February 2005 and will also see service in the FR-V from August 2005 onwards. All vehicles equipped with this diesel engine fulfil the Euro 4 Standard. In order to attain even greater environmental compatibility, Honda is currently developing an engine with a diesel particle filter (DPF) which considerably reduces the typical particle emissions for diesel engines. The CR-V fitted with this engine is to be made commercially available in the summer. The following will provide an explanation of the development concept for the diesel engine series i-CTDi as well as a technical outline of the adaptation for the diesel particle filter.

When developing the first diesel engine of the i-CTDi series, we initially aimed for application in the Accord. We formulated the following basic concept so as to progress beyond conventional diesel engines: ■ high propulsion power and at the same time low noise and vibration values through a lightweight and high-strength engine block ■ lower consumption and reduced exhaust gas emissions through optimised combustion and highly effective exhaust gas aftertreatment. This successful optimisation of the diesel engine also enabled heavier vehicles, of the SUV segment, such as the CR-V, to meet the Euro 4 Standard without DPF in a further stage. In order to attain as high an environmental compatibility as possible, the engine equipped with DPF was also installed in the CR-V in the third stage, whereby the following conditions were specified for the development: ■ The DPF system was to be integrated without discernible or perceptible affects in respect to the propulsion power, noise and vibration performance, driveability and fuel costs. ■ The basic development concept strived for was realised in the entire i-CTDi series.

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DEVELOPMENT

■ Vehicles which are fitted with these i-CTDi engines were to gain sufficient acceptance in the market. 3 Combustion Chamber and Materials

The Table shows the joint characteristics of the i-CTDi engine series. In order to find a suitable combustion chamber geometry with which both requirements of the basic concept can be realised at the same time, the homogenous and slow combustion was examined at various speeds and loads. Simulation calculations and measurements revealed that, in contrast to the Accord which uses a combustion chamber with exhaust gas recirculation, the diesel/air mixture would have to be more homogenous for the SUV model with its higher driving resistance in order to minimise the pollutant emission. For this reason, the ball-shaped combustion chamber was designed as a flat shell, while the diameter of the injection nozzle opening was reduced. The compression ratio was defined at 16.7:1, and the formation of a homogenous diesel/air mixture was aimed for without gaps in the piston crown through four vertically positioned valves. The injection pressure of 1600 bar is identical for the entire iCTDi series. Figure 1 shows a comparison of the combustion chamber and arrangement of the injection nozzles for the three development stages. Figure 2 shows the variation in time for the fuel volume determined from the CAE analysis in the range for which the mixture ratio within and outside the piston recess exceeds the value 2.5. Steps 1 and 2 both use a 6hole nozzle. Nevertheless, it was possible to realise a more homogenous mixture formation through a reduction in the hole diameter and a reduction in the piston recess depth, this leading to a reduction in the enrichment zone within and outside the piston recess, in particular in the last half of the injection period. The ASCT cylinder block (advanced semisolid casting technology) was used in all engines of the i-CTDi series, the overall view and casting core of which is shown in Figure 3. The ASCT method enables the production of high-strength, closed cylinder blocks through high-pressure casting technology, which are resistant to high temperatures and pressures. The ASCT method was devised while developing the i-CTDi engine for the Accord. In this process, the molten aluminium is injected into its mould in a semisolid state, whereby a higher casting quality is achieved in comparison to conventional die-casting on account of the thin property of the surrounding gas layer. This therefore

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also enables a heat treatment and hence higher strength of the material. For vehicles in the SUV segment with their typical properties such as high mass and frequent operation at high exhaust gas temperatures, cast steel is used for the exhaust manifold instead of the stainless forged steel used in the Accord, which enabled a high level of reliability to be achieved. In order to reduce the NOx (nitrogen oxide) emissions in the comparably heavy and large SUVs, the efficiency of the EGR cooler (exhaust gas recirculation) was increased while the diameter of the EGR valve was increased at the same time. This change lowered the temperature of the EGR gas and reduced the NOx emissions; nevertheless, the HC and CO emissions increased as soon as the combustion temperature was lowered too greatly. The EGR cooler was therefore fitted with a bypass and a valve which is controlled by the cooling water temperature. The NOx, HC and CO components in the exhaust gas can be reduced as a result of this. Figure 4 shows the EGR bypass system. The untreated emissions at constant partial load are shown in Figure 5. The measures mentioned above enabled the NOx emission and soot formation to be optimised. 4 Exhaust Gas Aftertreatment System

The structure of the after-treatment system is shown in Figure 6. Development stages 1 and 2 are defined by an underfloor dual-bed system which comprises a catalytic converter for the selective reduction of NOx as well as a pre-stage and post-stage for NOx cleaning by two different zeolites, whereby the temperature window was increased. In development stage 3, the oxidation catalyst or the particle filter designed as a catalyst carrier were located upstream, whereby the conditions for combustion of the accumulated particulate matter (PM) were optimised through oxidation, therefore enabling the overall particle emission to be reduced. In addition to this, the PM combustion conditions in the lower load range were also examined and a flap fitted for reducing the intake air volume in while a pressure differential sensor and sensors for the exhaust gas temperature were additionally installed for monitoring the soot formation in the DPF system. Figure 7 shows the system structure of the third development stage. Figure 8 shows the regeneration stage in the load diagram. In the area of the maximum load, the exhaust gas temperature rises sufficiently extensively and enables an automatic regeneration. However, PM re-

Diesel Engines

generation can also occur outside this range on account of the forced increase in the gas temperature at the DPF inlet (forced regeneration). At high loads, the intake air volume in is reduced and DPF regeneration enabled by controlling the VGT (variable geometry turbocharger). At medium loads, the gas temperature is not increased sufficiently by the VGT control alone. However, simultaneous valve opening and the post-injection associated with the exhaust stroke using the reaction temperature in the oxidation catalyst positioned directly below it enables regeneration of the particle filter at the same time. In addition to this, the gas temperature can also be increased in the lower load area by an additional second post-injection, thus enabling DPF regeneration. Nevertheless, the lubricating oil film in the engine is rendered thinner by this additional post-injection, hence reducing the viscosity of the oil. A limit value was therefore specified for the post-inspection volume in order to avoid this contamination – which is continuously monitored during operation, while the post injection volume is not continued beyond a certain value. This therefore ensures the reliability of the engine, as shown in the flow diagram in Figure 9, for regulation of the DPF forced regeneration. The PM accumulation can be calculated on the basis of the PM value and associated DPF temperature calculated from the engine speed, fuel volume injected and intake air volume in. If a definite amount of particulate matter has accumulated, the DPF forced regeneration is initiated and – as soon as the amount of PM collected has fallen to zero – deactivated again. 5 Summary

The developments described above make it possible to maintain the noise and vibration formation as well as handling characteristics at an optimum level during regeneration in CR-V vehicles equipped with DPF (development stage 3). Restrictions in the driving performance or, possibly, comfort are thus avoided. The performance of the i-CTDi engines is the same in all development levels, but an even better level of environmental compatibility was attained in stage 3. The relation between acceleration performance and fuel costs for vehicles of the CR-V series of stage 3 and the noise level attained in the interior confirm that the lowest noise and vibration values can be attained during low consumption driving. The engines equipped with a particle filter fulfil our basic concept for advanced diesel technology while exhibiting a high level of environmental compatibility at the same time. ■

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