Industry GASOlINE ENGINES

THE NEW MAZDA GASOLINE ENGINE SKYACTIV-G Skyactiv is a generic term for Mazda’s next-generation technologies being developed to achieve both driving pleasure and environmental and safety performance. It is a contribution to the company’s long-term vision for technology development. Of these technologies, this article describes the development of Mazda’s new highly-efficient direct-injection gasoline engine that achieves a compression ratio of 14.0 to 1. 40

AUTHORS

Tsuyoshi Goto

is Manager Engine Design ­Engineering Gr. No. 2 at Mazda Motor Corporation in Hiroshima (Japan).

Ritarou Isobe

is Manager Engine Performance ­Development Gr. No. 2 at Mazda Motor Corporation in Hiroshima (Japan).

Introduction

Mazda forecasts that internal combustion engines will still account for a high percentage of automobile powertrains even in 2020. In view of this, priority is given to the improvement in internal combustion engine efficiency as it offers the highest impact for environmental friendly vehicles in the markets. In Mazda’s opinion, whether a gasoline engine or a diesel engine, the ideal internal combustion engine to aspire to is the same. To reach this, Mazda has been adopting the advantages of a diesel engine to a gasoline engine, and vice versa, and enhancing the strengths of each engine, ❶. Through this challenge, Mazda newly developed the 2.0-l-Skyactiv-G as a gasoline engine and 2.2-l-Skyactiv-D as a diesel engine and plans to introduce them to the European market. Development Objective

Masahisa Yamakawa

is Staff Manager PT Technology ­Element Development Gr. at Mazda Motor Corporation in Hiroshima (Japan).

Masami Nishida

is Assistant Manager Engine Design Engineering Gr. No. 2 at Mazda Motor Corporation in Hiroshima (Japan).

The first step for Skyactiv-G to come closer to an ideal internal combustion engine was to improve its disadvantages relative to a diesel engine, namely the compression ratio (CR) and pumping loss, and to enhance its advantage regarding mechanical resistance. In light of this, Mazda set the overall target for the Skyactiv-G, aiming for a 15 % contribution to the improvement in the NEDC (New European Driving Cycle) mode fuel consumption and a 15 % increase in power output over the entire engine speed range compared to the current Mazda port

fuel injection (PFI) engine. To achieve this target, the technical objectives Mazda set for Skyactiv-G are as follows: :: increasing CR to 14.0 :: preventing increase in combustion duration even with a high CR of 14 :: reducing pumping loss by 20 % :: reducing mechanical loss by 30 % :: increasing the charging efficiency by 10 %. Problems to Overcome with High Compression Engine

Developing a high compression engine creates two key issues. Firstly, lower knocking resistance leads to ignition timing delay at high load, which reduces the engine torque. Secondly, there is a higher sensitivity for pre-ignition due to hot envi­ ronment and varied fuel octane number. Therefore, robustness against abnormal combustions (knocking, pre-ignition) was the key challenge to overcome. Power Loss Due to Knocking ❷ shows the power loss seen on a direct-

injection engine in the early development phase when the CR was increased from 11.2 to 14 with conventional technology. The ignition timing was significantly de­­ layed at 1500 rpm, resulting in a torque loss of 7 %. However, this torque loss value was smaller than Mazda initially predict­­ed. And even when the CR was increased to 15, the slope of torque loss curve was gentle, ②. Factor analyses re­vealed that weak exothermic reactions

❶ Vision for evolution of internal ­combustion engine 06I2011

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Industry Gasoline Engines

❷ Torque loss

took place be­­fore ignition because of the increased CR, and this minimized the decrease in torque. Mazda assumed that a high torque could be ensured even in a high compression en­­gine by resolving the knocking problems. Deterioration in knock resistance bases on gas temperature and pressure rise in the cylinders. ❸ is a numerical simulation which shows influence of both parameters. Lower in-cylinder gas temperature at the end of intake process is able to restore knock resistance characteristics which are deteriorated by the increased CR. When the CR is increased from 11.2 to 14 at 1500 rpm, the equivalent knock re­­ sistance can be ensured if the initial incylinder temperature (intake valve closed) is decreased by 35 K.

❸ Initial in-cylinder temperature representing equal knocking resistance

Key to efficiently cool down the initial in-cylinder gas temperature is to reduce both, working gas temperature by the latent heat of fuel vaporization and the amount of hot residual gas, which leads to the charging efficiency improvement with minimum deterioration in knock resistance. It is common understanding that rapid combustion is efficient to improve knock resistance [2], and Mazda has made following changes to achieve this. Improvement of Knock ­Resistance by Rapid Combustion

At project start, a conventional flat piston shape was used. For increasing the CR, the top of the piston was simply raised in a trapezoidal-shape, ❹. With this piston,

however, an even lower thermal efficiency was achieved. It is believed that the initial flame front hit the piston, which increased cooling loss and inhibited proper flame propagation. To resolve this, a hemispherical cavity was created on the piston top to enable initial flame to grow properly. With this change, the combustion duration (main combustion duration: 10-90 % mass burn ratio) was reduced by 1° CA at 1500 rpm. At the same time, knock resis­ tance improved, resulting in torque rise of approximately 2 %. For further improvement of the combustion duration, the entry angle of the intake port and valve fillet angle were optimized to intensify the in-cylinder flow, and the cylinder bores were made smaller than those of the current Mazda 2.0-l-engine (di­­ameter 87.5 to 83.5 mm) to shorten the combustion duration. This led to a further reduction of the combustion duration by 2° CA, improving the torque by another 4 %. Improvement of Knock ­Resistance by Latent Heat of Fuel Vaporization

❹ Measures on torque loss due to knocking

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In order to efficiently cool down the incylinder gas temperature by the latent heat of fuel vaporization, fuel pressure at 1500 rpm was raised to 10 MPa and a modified multi-hole (6) injector (MHI) with optimized penetration and spray angle was employed. The analysis of the in-cylinder temperature showed an im­­ proved temperature distribution through the split injection of the MHI and a re­­ duced initial in-cylinder temperature by 6 K. With this effect, knock resistance was im­­ proved, and torque was increased by 3.5 %.

tional 2.0-l-PFI-engines torque has been improved by more than 15 % over the entire engine speed range (over 10 % torque improvement when compared to the direct-injection engine with a CR of 11.2). Robustness against Pre-ignition

Especially under disadvantageous environmental conditions it is essential for high compression engines not to generate abnormal combustion such as pre-ignition. Therefore an advanced pre-ignition avoidance control system had to be de­­ veloped with focus on: :: influential factors (intake air temperature, humidity, coolant temperature, CR increase due to carbon build-up, octane number, etc.) :: effects of control factors (injection timing, intake valve closing (IVC) timing, etc.). Accordingly, Mazda established a control logic ensuring a sufficient safety margin against pre-ignition under various environ­ mental conditions. If, by accident, circumstances force minor pre-ignition, Skyactiv-G’s robust system detects a slight change in combustion pattern as a change of ion current, and it then enriches the air/fuel ratio and controls IVC timing in order to further prevent pre-ignition. Emission Reduction Technology

❺ Optimum exhaust system for scavenging effect

Improvement of Charging ­Volume (Torque) and Knock ­Resistance by Scavenging Effect

Mazda focused on improving the charging efficiency by scavenging the hot residual gas to maintain knock resistance and raise torque on a high compression engine. The current 4 into 1 exhaust manifold system offered 6.7 % of residual gas. A reduction to 3.9 % was aimed, which would decrease the initial in-cylinder temperature by 18 K and raise charging efficiency by 9 %. As per ③, this temperature decrease is consid­­ ered to be equal to the decrease of CR by 1.5, and this implies that the target torque can be achieved. When a high blowdown pressure arrives at the next cylinder during the intake and 06I2011

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exhaust valve overlap, the amount of hot residual gas increases, deteriorating the knock resistance. To avoid such condition beyond 2000 rpm, a 4-2-1 long exhaust manifold was applied. For adequate scavenging a high vacuum pressure wave is required during the valve overlap period. With appropriate exhaust system specifications for the targeted ex­­ haust pulse and controlled valve overlap period, a good scavenging effect can be achieved over a wide engine speed range, ❺. This improved knocking resistance and charging efficiency by 9 %, resulting in further torque improvement of 7.5 %, ④. ❻ shows the torque curve of the 2.0-l-Skyactiv-G and the improvements of the initial in-cylinder temperature and combustion duration. Compared to the conven-

The catalyst converter temperature of the 4-2-1 long exhaust system does not rise as rapidly as that of the conventional 4-1 short exhaust system with close-coupled catalyst (CCC), causing a delay in the starting time of exhaust gas purification. Furthermore, raw hydrocarbon (HC) increases with CR. It was therefore indispensable to develop advanced emission reduction technology that would meet even severer emission regulations of the future. This led Mazda to focus on both HC reduction at engine-start when HC level is high as well as the rapid catalyst converter warming after engine-start. As for the engine-start, Mazda raised the start-up fuel pressure from 0.43 MPa to 6 MPa. As a result, the HC at enginestart was reduced roughly by half. As for the rapid catalyst converter warming after engine-start, raising exhaust gas temperature by delaying the ignition timing is

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known to be effective. Excessive retardation, however, causes unstable combustion, limiting the rise in exhaust gas temperature. In the case of the Skyactiv-G, direct injection creates an ideal air-fuel mixture for combustion around the spark plug and generates lean stratified combustion. The lean stratified combustion and the raised combustion stability by increasing the CR led to a large improvement of the tradeoff between the gas temperature raised by ignition timing delay and combustion stability, ❼. Consequently, Mazda achieved rapid catalyst converter warming even with the 4-2-1 long exhaust manifold while maintaining stable combustion, achieving an emission reduction system that even meets the PZEV (Partial Zero Emission Vehicle) requirements.

❻ Improvement of torque, initial in-cylinder temperature and combustion duration

Fuel Efficiency ❽ shows the Skyactiv-G’s contribution to

the improvement in NEDC mode fuel ­consumption and the breakdown. A 15 % im­­provement has been made by the ­Sky­activ-G mounted on a medium-sized car compared to the current Mazda 2.0-l-PFI-engine. Effect of Increasing CR ­( Including Effect of Reducing Pumping Loss)

In the early development stage, the fuel consumption did not show any further im­­ provement when applying CRs higher than 13. The cause of this was the flat piston preventing the initial flame propagation. As a countermeasure, Mazda promoted the initial flame propagation by combining the hemispherical piston cavity and the intensified tumble flow, which is also an effective measure against the torque loss at WOT (wide-open throttle). The hemispherical piston cavity also prevented the hot initial flame from hitting the piston surface, which reduced cooling loss. Cooling loss was reduced further by adopt­ing the smaller cylinder bore, which largely im­­ proved thermal efficiency. Furthermore, a large reduction in pumping loss was made by utilizing the merits of increasing the CR. A conventional meth­­od to reduce pumping loss is to delay IVC timing and add external EGR gas. In this method, however, the reduction in pumping loss is very limited due

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to a decrease in combustion stability. Whereas with Skyactiv-G, the high CR secures an effective CR necessary for

combustion even with significantlydelayed IVC, and moreover, hot internal EGR (exhaust gas recirculation) gas is

❼ Improvement of catalyst converter warming after engine-start

:: Lubrication system: The major changes were the adoption of simple hydraulic routing to reduce pressure loss, minimization of oil pressure required by each hydraulic device to reduce oil discharge amount, and adoption of the electronically-controlled variable pressure oil pump to reduce oil pressure at light-load operation. :: Cooling system: The major changes were the reduction of resistance in the coolant passage to lighten work load of the water pump and adoption of the high-efficiency plastic impellers to im­­ prove work efficiency of the water pump. Summary

❽ Improvement in NEDC mode fuel consumption

­ tilized. This delivers the combustion as u stable as that of con­ventional PFI engines despite the IVC at 110° CA and the internal EGR gas increas­­ed by the extended valve overlap with use of a dual variable valve timing, ❾ above. Thanks to this, pumping loss has been reduced approximately by 20 %, ⑨ below.

high-stiffness straight guide and the reduction of chain tension through the stabilization of chain behaviour by the control arm on which load is evenly dispersed.

The Skyactiv-G unit is Mazda’s next-generation key engine that achieved 15 % im­­ provement in fuel efficiency and significant performance upgrade symbolized by 15 % increase in torque in the whole speed range compared to the current PFI en­­ gines. The goals for cost and weight were also achieved. Technical highlight is the concept of a natural aspirated gasoline engine with very high compression ratio of 14 for achieving best engine internal efficiency. Various technical measures were developed in

Reduction in Mechanical ­Resistance

In total, the mechanical resistance of the Skyactiv-G has been reduced by 30 % com­ pared to the current Mazda 2.0-l-PFI-engine. ❿ shows the reduction rate of each system achieved by the following changes: :: Crankshafts, pistons and conrods: The major changes were the reduction of the crank journal diameter from 52 to 47 mm while ensuring required stiffness, and improvement of LOC robustness, which achieved a 38 % reduction in piston ring tension. :: Valve train and chain systems: The major changes to the valve train system were the adoption of roller finger followers and valve lift load reduction by cam lift curve optimization. The major changes to the chain system were the reduction in friction between the chain and chain guide by employing a 06I2011

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❾ IVC timing delay and reducing pumping loss satisfying combustion stability

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order to overcome the issues, which until now prevented the application of very high compression ratios. With the introduction of 2.0-l-Skyactiv-G, Mazda meets the requirements of European customers for an environmental friend­ ­ly and economical gasoline engine together with further enhancement of the vehicle performance. This enables Mazda to offer a competitive power train for the coming new passenger car generation. References

[1] J. C. Livengood et al.: Correlation of Autoignition, Phenomena in internal Combustion Engine and Rapid, Compression Machines, Proceeding of 5 TH Symposium (international) on Combustion (1955), pp. 347 – 356 [2] Hisato Hirooka, Sachio Mori and Rio Shimizu: Effects of High Turbulence Flow on Knock Cha­ racteristics, SAE Technical Paper Series 2004-010977

THANKS An important contribution to this article was made by Ichirou Hirose and Hidetoshi Kudou, all who are involved in the development of the Skyactiv-G at Mazda Motor Corporation and members of Mazda Motor Europe.

DOI: 10.1365/s38313-011-0063-8

❿ Reduction in mechanical resistance

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