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Cummins for 21st Century

14th November 1996
Page 20
Page 20, 14th November 1996 — Cummins for 21st Century
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EXCLUSIVE

by Steve Sturgess • More is surfacing about Cummins' Apex—the double overhead camshaft diesel engine designed to take the company into the next century.

Prototypes on field trials in North America have cylinder heads cast by a foundry in Italy but production DOHC castings will be sourced in Mexico.

Early engines are said to be 15 litres and code named AHD (presumably for Apex, High Displacement) but it is believed that a 13-litre variant is in the pipeline. This would fit in well with the Cummins line-up, where the bigger Apex is tipped to produce 600hp (447kW) with 2,050Ibft (2,780Nm) from its launch, replacing the N14 highperformance engines.

The 13-litre will provide a useful link between the high performance Apex and the 11-litre MU when the N14 disappears, possibly in 1998.

CM understands that the Apex project has been working toward a March 1998 launch. That will pose an interesting question for Cummins in the US, where new emission standards come into force in January 1998. Will the company continue to sell the N14 using up banked and traded emissions credits until Apex is ready for the market three months later?

Although prototype engines confirm the layout of the camshafts, the cams are no longer expected to drive the valves and fuel system separately. Instead it is likely that they will share valve actuation and fuel pressure/control functions. One possibility is driving the exhaust valves and fuel system with one cam and the intake valves with the other.

This would allow Cummins to run the engine on a modified Miller cycle, using a nominal compression ratio of around 18 or even 211 Thermodynamic efficiency can be improved by increasing the expansion ratio. In conventional four-stroke engines the expansion and compression ratios are identical; In Miller cycle engines the expansion and compression ratios can be preset independently to give higher efficiency without an increase in damaging engine "knocking". The Miller cycle was patented many years ago by a US engineer; the patents have now expired.

The compression ratios provide high efficiency and emissions controls but present enormous problems at wide throttle openings when combustion pressures rise to the point where, according to one source, "there aren't enough bolts made to hold it together"

Instead, the camshafts are decoupled to vary the intake using a device similar to the

variable overlap timing used by Honda on its variable-valve-timing petrol engines. Opening the intake valve early reduces peak pressures allowing the engine to run with an exceptionally high compression ratio.

Derivative

The fuel system is expected to be a derivative of the two-rail system pioneered on the industrial Cummins QSK. In that system one fuel rail provides system pressure to vary the amount of fuel injected while the second determines the injection advance. The prototype Apex engines are clad with sheet metal to prevent prying eyes getting too close a view but early reports of running engines tend to back up the suggestion they are running a modified Miller cycle and a high compression ratio.

According to one source, who has seen and experienced the 600hp engine in a truck chassis, it is "mean and nasty". He adds that it idles like a top-fuel dragster and out-pulls anything currently available.

Other features of the Apex include an integrated Jake Brake, developed by Jacobs for

Cummins. However, there are some curious developments in the exhaust system. The manifold is known to be very short for good thermodynamic efficiency into the turbocharger.

A short exhaust path also means good throttle response and tuned performance.

However, it may mean that cylinders one, two and three have the two inlet valves oriented to the front of the engine and four, five and six have them oriented to the back, with cylinders three and four featuring siamesed exhaust ports, perhaps separated by a sheet-metal baffle.

Whatever the valve layout, it seems that the new Cummins engine platform will include a great deal of new technology. It also sounds like fun to drive.

Silencing the exhaust of the high compression engine may prove a challenge, but for those who like such things it should sound really wicked with a pair of six-inch straight pipes...

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