Released in Australia at the 2004 Sydney International Boat Show, the Volvo D9 adds to the already innovative D-series of electronically managed marine diesels, starting with the D3 and currently ending at the D12.

The D9 is a straight six, which gives the best inherent engine balance of any number of cylinders except perhaps at V12.

As with its D6 counterpart, the D9 has OHC valve actuation and four valves per cylinder, but unusually for its piston displacement category it has unit injectors instead of common-rail injection. With European manufacturers such as Iveco and MTU switching to common rail across their ranges, the D9 has more in common with its Cat competition.

Cat Marine claims IMO marine diesel engine emissions compliance for the C9, but Volvo Penta goes one further by stating that its D9 models meet EU and US emission levels for 2006/7.

Displacing 9.4lt with cylinder bore of 120mm and a piston stroke of 138, the D9 is available in two shaftdrive-only outputs. Based on 1.0hp = 746W, the less powerful D9-500 version develops 493hp at 2600 revs, while the D9-575 develops 567hp at 2500 revs.

Volvo Penta has obviously released the less powerful one so it can compete with engines such as IVECO's common-rail Cursor 500, which develops 493hp at 2600 revs from 7.8lt, but at 1075kg bobtail the D9-500 is 18 per cent heavier.

Still, it produces 1630Nm of torque at only 1400 revs compared to 1520Nm between 1800 and 2000 revs, and for torque-to-weight the D9-500 is not that far behind.

As for bulk, the Cursor 500 has length by width by height dimensions of 1671 x 956 x 943mm compared to 1550 x 858 x 1008mm for the D9-500; and apart from its additional height, the latter can fit into the same space as a significantly smaller displacement diesel.

MORE POWER & TORQUE
But the real eye-opener is the D9-575, which produces 1955Nm at 1700 revs from the same dimensions and weight as the D9-500.

Compared to the 8.8lt Cat C9, which develops 503hp at 2500 revs but 1867Nm at 1800 revs, the D9-575 is a more powerful engine. However, the bobtail weight is 14 per cent greater than the C9's 947kg, so the power-to-weight ratio is 0.5 per cent better - but the torque-to-weight ratio is eight per cent lower. The bobtail dimensions of the C9 are 1497 x 974 x 983mm.

Both engines are rated on Aussie-grade distillate density of 839gr/lt and don't require de-rating to operate satisfactorily on our substandard fuel. And both are rated for a maximum yearly usage of 250–1000 hours, with one hour of Wide Open Throttle operation in every 12 for the C9 and one hour in every eight for the D9-575.

So how do the two engines compare for maximum torque across the rev range? At 1000 revs the D9-575 produces 1110Nm compared to 938 for the Cat, 1520 versus 1200 at 1200 revs, 1840 versus 1722 at 1400, 1930 at 1600 revs versus 1854 and 1955 versus 1867 at 1800 revs.

The D9-575 produces its maximum torque between 1700 and 1900 revs, and with its rapid torque rise from 1000 revs, the D9-575 is able to handle heavier hulls far more effectively than the C9.

By 2000 revs the D9-575's torque has dropped to 1900 revs versus 1800 for the Cat; at 2200 revs the outputs are 1810 and 1628; at 2400 revs 1680 and 1492; and at WOT 1620 and 1423Nm.

RATINGS AFFECT FUEL CONSUMPTION
Where the prop-horsepower versus fuel-consumption ratings on these engines differ significantly is that Volvo Penta rates the average fuel consumption for planing hulls and semi-planing hulls, whereas (along with Yanmar) Cat Marine rates it for displacement hulls only.

As I mentioned in the recent Yanmar 315 review, approximately 10 per cent may have to be added to the fuel consumption where the hull passes the planing "hump".

At 1000 revs the D9-575 consumes 10.5lt/h compared to 8.1 for the C9; 17.5 at 1200 revs versus 12.1; 27 versus 20.9 at 1400 revs; 32 at 1600 revs compared to 33.5; 42 versus 42.2 at 1800 revs; and 57 versus 51.9lt/h at 2000 revs. However in the 1800–2000 rev-range, where these engines are most likely to plane a gameboat, the C9's consumption may rise to 46.4 and 57.1lt/h.

At 2200 revs the consumptions are 72 and 64.7lt/h; at 2400 revs 95 and 85.5; and at WOT 103 and 99.2lt/h. So for an increase of 13 per cent in power and 14 per cent in torque, the D9-575 consumes four per cent more fuel.

SOLID CONSTRUCTION
In essence a smaller version of the D12, both D9 versions have ladder-frame cylinder block construction and one-piece cylinder heads with the electronically controlled high-pressure unit injectors (gear driven from the timing gear) located centrally atop the combustion chambers between the two intake and two exhaust valves for each cylinder.

Replaceable valve guides and wet cylinder liners are fitted, and unusually the camshaft drive is at the aft end of the engine, simplifying attachment of PTO equipment at the forward end. The aluminium pistons each have one oil scraper and two compression rings.

Volvo Penta offers a choice of three single-speed hydraulic gearboxes with electronic gearshift control - one from ZF (down angle) and two from Twin Disc.

The new EMS 2 (Engine Management System) fitted to both engines was developed to meet the upcoming emissions requirements. It monitors and controls additional engine functions with fuel requirements, for example, monitored up to 100 times per second - and with more precision than a common-rail system, because each unit injector can be controlled individually depending on cooling-water temperature variations, for example, from one end of the engine to the other.

The EMS 2 works in conjunction with Volvo Penta's existing EVC instrumentation, which handles engine throttle and gearshift, and compensates for variations in fuel temperature between five and 55°C - but not variations in fuel density as does the common-rail D6.

The standard electrical system includes a 12V 115amp voltage-regulated alternator and a 24V 80amp alternator is optional, as are EVC monitoring panels for single and twin installations (the latter has automatic engine synchronisation) and complete instrumentation including digital fuel flow and range-to-empty.

For more information on both Volvo Penta D9 models, contact Graeme Avers at Eastern Engine, tel (07) 3902 5444 or email graeme.avers@volvopenta.com.au.