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01/08/2008
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Common rail technology has brought greater efficiency and refinement to millions of small diesel engines over the past 10 years – and there is more to come, says Andrew Noakes
 Although diesel engines have been used in passenger vehicles since the 1930s, it took the advent of electronic engine management in the 1980s to allow them to compete in earnest with spark-ignition (SI) engines for passenger car sales. The thermal efficiency advantages of the compression-ignition (CI) engine have never been in doubt, but for passenger car applications there had always been issues with the CI engine's perceived lack of refinement, particularly during cold starts and at idle. The stigma of association with commercial vehicles and taxis turned buyers away, particularly in premium sectors. The rapid growth of diesels in passenger cars since then – they now account for more than half of European car sales – is largely due to developments in injection technology. Engine power, fuel consumption, emissions performance and refinement have all made immense strides in a short time.
Compression ignition combustion demands high compression ratios, as the charge air temperature must be raised sufficiently high to ignite fuel. The high compression ratio naturally leads to high thermal efficiency. CI engines burn a heterogenous mixture which always has excess air, and the power output of the engine is controlled by the quantity of fuel injected. By contrast conventional SI engines burn a homogeneous mixture and as a result are tied to a relative small range of combustible air/fuel ratios, which means engine power must be regulated by adding a throttle valve to the intake, to control the amount of combustible air/fuel mixture reaching the cylinders. The use of a throttle inevitably leads to high pumping losses. CI engines consequently have a considerable efficiency advantage, particularly at part loads where road vehicle engines operate most of the time.
Early diesel engines used inline piston-type injection pumps, which remain in use today only on large commercial vehicle engines. A significant advance came when Bosch developed the compact VM 'distributor type' injection pump in the 1960s, but the real breakthrough for passenger car diesels came with the introduction of electronically controlled injection pumps in 1986. Sensors delivered information on accelerator pedal position, engine speed, intake air pressure and the temperatures of the intake air, coolant and fuel to an ECU which contained a series of predetermined control maps. Both fuel quantity and injection timing were under electronic control, providing more precise control of fuel delivery. Engine power improved while fuel consumption and emissions fell. Electronically controlled diesels also benefited from slower, smoother idle – an area where direct-injection diesels in particular struggle to meet customer expectation.
The introduction of the Bosch Unit Injector System (UIS) brought further improvements. UIS integrated a small high-pressure pump into the injector for each cylinder, normally operated by a rocker arm from an overhead camshaft. Injection timing and quantity were controlled by a solenoid valve commanded by the engine control ECU. While the idea of providing a dedicated injection pump for each individual cylinder was nothing new – it was standard practice on large industrial diesel engines in the early 20th century – the integration of a small pump into the injector in UIS brought significant benefits. Because the unit was compact and hydraulically efficient, peak injection pressures of up to 2,200 bar could be generated. Supplementary electronically controlled pilot injection in the low speed/low load range could also be implemented, to improve cold-start performance and reduce combustion noise.
In all of these systems, the maximum injection pressure was directly related to the engine speed and the quantity of fuel injected, which is less than ideal. High injection pressures are desirable because they improve fuel and air mixing throughout the engine speed range, leading to more complete combustion and the reduction of particulate emissions – but with the side-effect of an increase in NOx emissions requiring aftertreatment. Decoupling injection pressure from engine speed required a fundamental change in injection system design, separating the pressure generation and injection timing functions of the existing injection pump or unit injector into two individual stages.
This was achieved by the accumulator or 'common rail' diesel injection system, originally developed by Fiat and Magneti Marelli. The rail which gave the system its name carried high-pressure fuel, delivered by a single pressure pump, to all the injectors. Injection timing and metering of the fuel quantity were determined by the ECU and controlled by solenoid-operated injectors, as in UIS injection. The first-generation common rail system (CRS), introduced on the Alfa Romeo 156 JTD and Mercedes C220 CDI in 1997, used a simple pressure-regulating circuit to control fuel pressure in the supply rail. In later systems this was augmented by a pump control system which matched pump delivery to the amount of fuel injected, reducing the recirculation of excess fuel through the hot engine bay and as a result reducing fuel temperatures.
A further refinement was the introduction of piezo injectors, which were significantly faster in operation than existing solenoid injectors. As a result, the piezo injectors could deliver multiple injection events during each combustion cycle with much greater precision, allowing the use of pilot injection (to reduce noise and NOx) and post-injections (to minimise soot by extending burning into the exhaust stroke). The piezo injectors were also smaller, potentially reducing the overall height of the powertrain and allowing easier positioning of the injector in a modern four-valve combustion chamber.
Bosch describes its latest CRS as a fourth-generation system, which features a new CP4 pressure pump designed to deliver injection pressures of up to 2,000 bar. These very high injection pressures, delivered first by UIS injection and more recently by developed common rail systems, have almost rendered obsolete pre-chamber diesel combustion systems, which offered excellent mixture formation but higher heat losses and therefore higher fuel consumption. For passenger cars the focus is now firmly on direct injection systems, using multiple injection and exhaust gas recirculation to reduce the harsh combustion noise often associated with direct injection which initially limited it to commercial vehicles.
Today the best direct injection CI engines can largely meet SI standards of refinement, and as a result sales in Europe have grown enormously. When common rail technology was first brought to market in 1997, 22% of cars sold in Europe were diesels. Now diesels account for half of all European passenger car sales, with the highest share of sales in Belgium (74%), France (71%) and Spain (68%). Bosch's production of common rail diesel injection systems exceeded eight million units in 2007.
Although CRS is now firmly established in Europe there is massive potential for expansion elsewhere, with the US remaining the biggest potential growth market in the medium term. While vehicle fuel prices in North America are still considerably less than those in Europe, they have more than doubled since 1990 and the rise has stimulated market interest in more economical vehicles. Diesels offer high torque, mimicking in that respect the large-displacement gasoline engines American consumers have traditionally chosen, but offer much better fuel economy. Currently diesel-engined cars and light commercials make up around 5% of US vehicle sales (around 800,000 units annually) but Bosch expects diesel's market share to triple to 15% by 2015, supported by a fact-based marketing campaign which aims to bring the benefits of diesel to the notice of American consumers.
Common rail engines will also drive diesel growth in the emerging Asian markets. Bosch predicts annual sales of around 1.3 million units in China and around the same number in India by 2010, with sales continuing to increase as both markets develop. Suppliers of diesel injection equipment are already setting up production facilities in these areas. Bosch has established plants at Nashik and Bangalore, manufacturing injector components and high-pressure pumps respectively, while Denso expects to begin production of CRS equipment in China in 2009. With these markets in mind Bosch is developing low-cost common rail systems with injection pressures in the 1,100-1,450 bar range for 'low price vehicles', costing below E7,000. It expects such vehicles to make up 13% of the world passenger car market by 2010, a volume of around 10 million vehicles a year.
While low-cost CRS is the focus of development for emerging Asian markets, the more mature European and North American markets present different challenges imposed by tightening emissions legislation, and in particular the significant reductions in allowable emissions of nitrogen oxides (NOx). NOx is produced during combustion when airborne nitrogen reacts with oxygen, which occurs readily when very high temperatures are generated during combustion. Euro 5 due to come into force in 2009 reduces the NOx limit from 250mg per kilometre to 180mg/km, and the 2014 Euro 6 standard will reduce that still further to just 80mg/km.
Methods of meeting these standards using a combination of improvements to the diesel combustion process have already proved successful in commercial vehicle diesel engines. These include the use of higher charge air pressures through more aggressive use of turbocharging, higher rates of cooled exhaust gas recirculation (EGR) and higher injection pressures at part load, and using high-speed injectors to deliver up to eight separate injections in each combustion cycle. Promising recent research in this area by Delphi has seen test engines deliver low NOx and soot emissions without a fuel consumption penalty by significantly advancing the injection timing at part load, producing a prolonged pre-mixing phase, then increasing swirl and injection pressure to reduce soot formation.
This is a form of pre-mixed charge compression ignition (PCCI) which is a halfway house between existing diesels and the potentially even more effective homogeneous charge compression ignition (HCCI), which is being developed by several OEMs and their suppliers. In HCCI a combustible mixture is compressed to the point where it self-ignites, in-cylinder conditions being controlled by varying the rate of EGR, the valve timing and, in one design, the compression ratio to control the timing of ignition. Although the system can keep NOx levels low, it can produce unacceptable levels of HC and CO, and costs are high. The technology is still in its early stages of development, and it is too soon to say whether it will definitely achieve all that is hoped of it.
While research continues into more advanced combustion techniques, careful development of diesel injection can meet the requirements of the Euro 6 test, but the tighter US standard seems certain to require the use of aftertreatment in the form of urea injection and a selective catalytic reduction converter, which assists the conversion of NOx and urea into nitrogen and steam. Though this system works well it remains to be seen whether consumers will ensure the liquid urea system is refilled during scheduled maintenance or will instead take the cheaper option of allowing it to remain empty and inactive. The latter seems likely unless drivers are required by law to maintain their vehicles correctly. Changes to the emissions tests which are part of mandatory annual vehicle inspections might be necessary to ensure these systems continue to provide the emissions performance they are capable of when new.
The future for diesel in the short to medium term seems likely to develop in two different directions. In more sophisticated markets such as Europe and the US the emphasis remains on maintaining power and refinement, while reducing fuel consumption and raising emissions performance. This will lead to ever higher injection pressures, more complex multiple-event injection strategies and innovative combustion modes such as PCCI and HCCI to further reduce combustion noise and combat NOx and particulate soot emissions. In developing markets such as China and India the challenge will be to retain the fuel consumption and emissions benefits of sophisticated diesel injection while using lower injection pressures and reducing the unit cost of the system so it can be applied to the very low-cost vehicles which will make up the bulk of sales in these markets. In both cases, developments of CRS injection seem certain to play a major role.
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Author
Andrew Noakes
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Copyright European Automotive Design.
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