Scania's New High-Tech 470 Engine

Diesel Progress North American Edition, Nov, 2001 by Peter Mullins

Editor's note: As exhaust emissions standards become more stringent across the globe, diesel engine manufacturers have developed a range of strategies to achieve compliance. This engine, detailed by Diesel Progress International Editor Peter Mullins, shows how one major European manufacturer plans to reach that goal.

Earlier this year, Scania introduced a new 470 hp Euro 3 version of its 12 L six-cylinder truck engine. The power unit, designated appropriately the 470 and aimed at trucks between 40 and 60 metric tons capacity, is equipped with new turbo and fuel technologies that boost efficiency and make for quiet and relaxed driving. Leif Ostling, Scania president and CEO, explained that truck operators are demanding more performance and driveability and the technologies developed for this engine will eventually be applied to other engines in the company's portfolio.

Prime features of the 470 engine are a new high-pressure fuel injection (HPI) system operating at approximately 23,206 psi (with the potential to go much higher) and an energy recycling concept called turbocompounding.

The Scania HPI system is a cooperative development with Cummins which has taken some two years of fine tuning to perfect. The 470 is the first Scania product to use it although it has been employed on Cummins K19 industrial engine models as well as the 15 L, 600 hp Signature truck diesel and its lower-rated ISX derivatives. The system is being manufactured alongside the Signature production facility in a jointly owned plant in Columbus, Ind., in which Cummins has a 70 percent interest.

One of the HPI system's key features is the electronic control of injection timing, which is a major leap forward on the former STC (stepped timing control) installed on Cummins 14 L nonelectronic engines. Two separate passages carry fuel to each HPI injector. Both basically fuel the engine, but they each have individual functions: one to control the timing, the other to control the pressure on which metering depends. This is effected by a control valve regulated by the electronic control module.

Two solenoid-actuated spool valves inside one valve block control the pressures of the two fuel outputs, which are then fed to separate timing and metering chambers in each HPI injector. The volume of fuel in these chambers is controlled by the injector's cam-operated top spindle acting on two floating plungers below it. The upper space emulates STC's hydraulic link, although using fuel rather than the lube oil employed by the STC system. The current HPI injector technology gives continuously variable injection timing under electronic control.

Scania pointed out that in restricting the injection operating pressure to 23,00 psi, it is being very conservative. The system has the potential to go to more than 28,000 psi and possibly as high as 34,000 psi. This means that future uprates of the 470 engine equipped with HPI injection, turbocompounding and possibly exhaust gas recirculation (EGR) to reduce [NO.sub.X] will be able to meet Euro 4 emissions legislation in five years time.

Turbocompounding, for its part, is not exactly brand new; Scania pointed out that it started as a joint development with Holset, Cummins' turbocharger arm, some 10 years ago. In fact, the system was applied to a number of Scania power units. However, at that time emissions were not that serious a problem and Scania's conventional engines in this power bracket were fully adequate to meet market demands. Using a second turbocharger to extract residual energy from the exhaust gases and recycle this in the form of mechanical energy to the crankshaft was difficult to justify at that time.

There were also technical considerations. Before electronic control became widely available, turbocompounding proved worthwhile only under high load. In European driving conditions with constant gear changing, fuel consumption was no better than with regular turbo intercooling; the limitations resulted more from the mechanically controlled fuel pump than with the turbo system. The advent of HPI provided the flexibility to match fuel input with injection timing to produce the precise combustion conditions for fuel economy and low-rev torque performance.

The 470 turbocompounding system comprises a Holset HX55 turbocharger and a Holset HP72 power turbine designed and developed specifically for the 470 engine. The system differs from conventional turbo charging in that a secondary turbocharger is installed in the downstream exhaust flow. This large slower-speed turbine extracts energy from the exhaust that would otherwise be wasted and transfers this back to the engine output shaft via a high-speed transmission system.

This results in improved fuel economy and engine performance and also helps to reduce noise from the vehicle. The low-inertia turbocharger and power turbines are designed as a matched pair to ensure optimum system efficiencies. A high back-shroud radial turbine delivers a peak turbine efficiency approaching 80 percent, the company said. The power turbine also features an unusual single-piece shaft bearing for improved gear end load bearing and ease of manufacture. The HX55 turbocharger comprises a standard compressor stage with a bespoke small-- area radial turbine.