With excellent performance and surprising fuel economy, Volvo's KAD and KAMD series super and turbocharged diesel powerplants offer the best of both worlds

Regular watchers of Australian drama series Water Rats may recall the 'song' of the Volvo diesels in the police launch Nemesis as it accelerated onto the plane.

The tune comes from the unique combination of forced induction featured on the maker's KAD/KAMD series diesels. Put simply, these engines combine a belt-driven mechanical supercharger (Kompressor in Euro-speak) and an exhaust gas-driven turbocharger.

By way of explanation, the infectious 'whine' you hear when Nemesis heads off to save the day is the supercharger spooling up and forcing air into the combustion chambers to improve out-of-the-hole performance. With conventional turbodiesels, there's often a delay (or lag) before the turbo comes on boost...

Volvo Penta's direct-injection Kompressor models were released locally a few years ago with the six-cylinder 3.6lt 42 Series. Since then the powerplants have been steadily upgraded and the range expanded.

The maker now offers three engine configurations: the eight-valve 170hp 2.4lt four-cylinder KAD32; 230hp 12-valve KAD43 six; and the 24-valve 'flagship' the Electronic Diesel Control (EDC)-equipped KAD44EDC.

KAD signifies that the powerplant is equipped with Volvo's excellent Duoprop sterndrive leg. Shaft-drive fans have not been forgotten the 43 and 44 Series powerplants are available in KAMD versions fitted with a selection of reduction gearboxes or V-drives.

In the case of the KAD43/44, the relatively simple 3.6-litre straight six base engine has been retained though significantly upgraded from its 42 Series days. Unlike most automotive diesels of this size it's features pushrod valve actuation unusual given that the 44 also boast four valves per cylinder.

Like the rest of the KAD/KAMD range the 44's induction system is basically very simple, comprising a belt-driven air compressor in addition to the normal turbocharger. When the throttle is rapidly opened the mechanical compressor is engaged by an electromagnetic clutch, compressing air and forcing it through the turbocharger, creating more torque at lower revs. Once the turbocharger has reached its normal boost pressure (at about 2400rpm and just below maximum torque) the compressor is disengaged and the turbo takes over. A non-return valve in the intake manifold prevents air from passing through the compressor once it has been disengaged.

When the revs fall below about 2400rpm, the compressor cuts in again as the turbocharger starts to fall off boost, keeping the overall boost pressure fairly constant. Volvo Penta says this enables a KAD-equipped boat to maintain a clean plane at lower revs and reduces fuel consumption and black exhaust smoke from fuel oversupply at low revs under load.

Thus unlike conventional supercharged engines where the compressor is always working and drawing power from the engine, Volvo's Kompressor only cuts in when needed.

Some owners I've interviewed complain about the whine it produces at low planing speeds. To be honest it's a small price to pay for such good performance across the entire rev range.

According to Volvo, which tested twin 5.7 litre V-eight sterndrives against its original 3.6 litre 42 series in the same type of boat, not only did the Kompressor engines outperform the V-eights at higher speeds but also beat them for out-of-the-hole acceleration.

The 44 Series EDC further improves output and fuel efficiency by ensuring that the injection timing is accurate at all revs, including the period when the turbocharger takes over from the supercharger.

This electronic engine management system overcomes timing that is either too retarded or advanced above or below the maximum torque area of conventional fixed-timing diesels. The system also compensates for high fuel temperature to overcome power loss in hot climates and has a self diagnostic facility to reduce servicing time (and costs, down the track).

The engine always operates at peak efficiency, much like a fuel-injected car.

A side benefit is that shifting from neutral to ahead or astern is smoother as the EDC maintains a lower, more stable idling speed than fixed-timing diesels. And as the shift control is electronic there are no control cables to send engine vibration through to the helm station.

Owner-operators of EDC Volvo-equipped trucks have informed me that they are 40-50% more fuel efficient than conventional fixed injection-timing diesels.

The KAMD43P (without EDC) develops 230hp at the crankshaft and weighs 534kg and the KAMD44P EDC develops 260hp and weighs just 4kg more.

As for torque, no standard V-eight inboards come close. The Volvo produces about 650nm, while the top-of-the-line 400hp MerCruiser 8.2 litre MPI, which weighs 499kg, develops 550nm.

According to Volvo, its KAMD44 EDC consumes about 60lt/hr at WOT, compared to 110-120lt/hr for the MerCruiser V-eight -- a big saving for owners of canyon runners and charter operators.

The above mentioned four-cylinder KAD32P weighs 475kg, while the KAD43P weighs 570kg and the KAD44P EDC 572kg. All weights include Duoprop leg.

Apart from greater mechanical complexity compared to conventional turbodiesels, the Kompressor system adds weight and like its variable injection timing MerCruiser diesel competition, the Kompressor series are considerably heavier than Yanmar's lightweight turbodiesel series.

For example, the inboard KAMD44 EDC is comparable to the 240hp (propshaft rated) 4.2lt MerCruiser D-Tronic turbodiesel at 551kg. The 250hp (crankshaft) 4.2lt Yanmar 6LP-DTZE weighs just 380kg, a weight saving that could overcome the OEsofter' throttle response at low revs.

Complete with Duoprop sterndrive the Volvo weighs 572kg, while the MerCruiser with Bravo Three leg (dual-prop) weighs 588kg. So equipped, the Yanmar weighs 498kg, a significant weight saving down aft!

Apart from servicing the mechanical compressor and electromagnetic clutch and ensuring valve clearances are to factory specs, maintaining the Kompressor series of diesels should be no more complex than conventional turbodiesels.

Always use the right grade of diesel fuel (securing this from commercial outlets such as fishing co-ops should reduce the risk of picking up fuel OEcut' with heating oil) and change the oil and filters according to Volvo's recommended schedule.

Remember that oil is the lifeblood of any four-stroke engine and as turbodiesels stress lubricants far quicker than naturally aspirated units, you must change the oil and filters strictly according to the recommended schedule.

As a professional engine tester who specialises in evaluating low-pollution motors, I love the concept of Volvo's Kompressor series, particularly the EDC-equipped 44. And though these motors are a direct result of Volvo having to comply with tough emission laws such as the Bodensee (Lake Constance, Germany) regulations, it's refreshing to see the approach taken to create low emission diesels that perform well yet are relatively straightforward to service!

Published : Friday, 1 September 2000

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