V olvo's new 12-litre engine was launched in the FH range
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last September amid all the hype and publicity corporate PR could muster. Talk of a £600m development budget filled the conference hall as Volvo became the first manufacturer in recent years to launch a new cab and engine in the same vehicle.
For the operator it could be the beginning of a new chapter; for the development team in Sweden it was the end of the story. After seven years the job was signed off. So why did Volvo scrap the 12-litre engine so loved by operators around the world?
Because it had to. With a basic design dating from 1965, Volvo's engineers concluded that the 12-litre had inherent faults which would prevent it meeting future emission legislation.
The existing 12-litre had met Euro-1 by using electronic diesel control. But Volvo was looking at Euro-2, Euro-3 and even Euro-4 (it predicts 5NOx and 0.1g/kWh of particulates for Euro-3; 3NOx and 0.1g/kWh for Euro-4). So the old 12-litre had to go and Volvo's engineers turned their attention to its replacement.
There is an Irish saying: "If I was going there I wouldn't start from here." Everyone laughs but knows exactly what it implies. Volvo found itself in that position about seven years ago when it took the decision to scrap the old 12-litre and, in effect, started the design with a clean sheet of paper.
Volvo defined the parameters for its new engine simply: world-class fuel economy and a platform for future environmental development. In addition, the company felt it was important that its engines should not emit transient black smoke or white smoke during a cold start—but should still reach 90% of torque within two seconds. To achieve this each area of concern was addressed separately.
As the need to meet predicted emission legislation killed the old 12-litre, minimising emissions was the logical place to start with the new design.
About 50% of particulates is soot (another 20%, approximately, comes from lube oil and fuel), so the easiest way to reduce particulates is by minimising soot. Increasing injection pressures makes the diesel droplets finer, allowing more complete combustion and minimising soot formation. But the injection pressure increase available from traditional pumps is limited by the pipes swelling as the fuel pulses down them.
Reducing NOx is most easily achieved by retarding the timing, but doing so increases fuel consumption (we have seen some evidence of this when comparing Euro-1 engines with their less-green counterparts).
With increased injection pressure the amount of swirl in the combustion chamber needs to be reduced to stop the finer droplets being thrown against the cylinder wall by the swirling air.
Combustion chamber
Improving the fit between piston and cylinder would cut the amount of oil getting past the rings into the combustion chamber. This requires a robust engine design to minimise distortion during the cycle (and helps keep noise down). Pumping losses must be kept to a minimum to prevent a rise in fuel consumption, and the inlet air should be as cool as possible.
When all the demands were summarised (see diagram) the basic construction of the new engine started to suggest itself, Electronic unit injectors allowed increased injection pressures and flexible timing. Getting the most out of the system required rapid cam rises (to get maximum fuel pressure), placing considerable stress on camshaft, followers and pushrods. Moving to an overhead camshaft overcame these problems, but increased the height of the engine.
Lower swirl requirements meant that both the cylinder head and piston crown had to he redesigned.
Switching to a four-valves-per-cylinder layout during the redesign reduced pumping losses, gave a lower inherent swirl and allowed for a central, vertical injector. It also improves the breathing of the engine by around 100O.
Steel piston
Due to its lower coefficient of expansion (similar to that of the liner), the articulated steel piston allowed a closer fit with the bore than all-aluminium designs, says Volvo. This minimised oil penetration of the combustion chamber, while the new piston had its top land reduced to minimise the dead area above the top ring between piston and liner (well known for producing emissions).
With a stiffer engine design, a higher compression ratio was possible, which minimised the required timing delay: reducing NOx and noise while maximising fuel economy.
Why has it taken so long? Again it's the "I wouldn't start from here" problem. With a new engine block, pistons, head and injection system to design, develop and optimise all at the same time the permutations seem endless.
From experience, Volvo began with the pistons. Having opted for articulated pistons, computer modelling was used to get the three basic bowl designs to use with the lower swirl.
The piston crown was only one side of the swirl equation; the other is the incoming air through the cylinder head. As far back as 1988 the first head design was under test, with four valves per cylinder and a central injector. While computer modelling helped narrow down the variables, trying a range of settings around the most promising permutations, was unavoidable.
Combinations of three swirl levels, piston bowl designs and injector protrusions at four compression ratios all had to be evaluated. To simplify the process a cylinder head was designed for this engine with a variable inlet port to allow a range of swirl levels lobe tried.
As is usual the initial tests were carried out using a single-cylinder engine to reduce costs, complexity and other variables. The test sequence was for each set-up to be run at four or five timings on a cycle covering 15 to 20 speed/load points—that adds up to more than 500 tests. On each test the power, torque, fuel consumption and emissions were monitored and logged. This part of the process took up a massive amount of time, manpower and money. And if, on collating the results, say a compression ratio between two existing settings was identified as the optimum, then a series of tests has to be run at that setting.
Computer model
t d t hi wen roun particular loop three times but it was starting from scratch with a new injection system.
Having got the basics about right, fine tuning of areas such as injector hole size could be carried out—this had more effect on emissions than performance.
With the combustion process reasonably well defined, the mechanical components of the full-size engine could be sorted out. In the move from a single cylinder to an in-line six items such as castings, timing gears, bearings and seals all needed careful design work. The engine had to be be economical to produce using modern techniques and it must not break, leak or wear out too quickly—the lifespan of the engine had to be established in field testing. All the tests in the lab or computerised finite element analysis could never simulate the demands made on the engine by differing drivers and traffic conditions.
The road-going stage of the work began almost live years ago with prototype engines running in vehicle fleets. These were housed under the F cab—the FH cab was being developed at the same time.
Engines were measured before assembly and put in trucks for fleet testing. Fuel and oil consumption and driver comments were carefully monitored while any problems were investigated. After 300,000km on the road the engines were taken out of the trucks and performance tested on the bench before being stripped down. Any defects were noted and the original components re-measured to assess wear rates.
Again modifications were incorporated where needed, along with those from the fine tuning in the lab before further road trials were undertaken. The new cab and the new engine development programmes accounted for around 15 million km of road testing. .
But this is not the end of the story. Volvo predicts an increase in the pace of development, with new emission standards to be met every four years for the next 20 years.
The company is confident that its new engine will be able to meet future emission legislation for the next 10 years or so. By the end of the decade, Volvo predicts, exhaust gas recirculation (EGR), wastegated turbochargers and water injection will be commonplace and emission limits will be down to 2gm/kWh of particulates and 0.1 NOx. But meeting those standards could involve up to 15 years of development work and another £600m—or more.
E by Colin Sowman