Volvo Penta D2-75

Released at the 2003 Sydney Boat Show, the 2.2lt four-cylinder D2-75 is a turbocharged and aftercooled version of the naturally-aspirated D2-55 released locally a couple of years ago.

It has all the features of this engine, such as a cast iron cylinder block and head and a five main bearing crankshaft with integral counterweights running in a rigid tunnel block crankcase. The high silicon aluminium pistons have two cast-iron chromium-faced compression rungs and a single oil control or scraper ring, and replaceable hardened valve seats are fitted.

As with its D2-55 counterpart, the D2-75 has indirect injection - a feature I'm not so happy with in turbo-aftercooled diesels. Admittedly the D2-75 does run out to only 3000 revs, so cracks around the pre-combustion chambers should never occur - and in order to meet EU and US EPA 2006 oxides of nitrogen limits without electronic engine management, indirect injection was the only choice. At 23.3:1 the compression ratio is very high, so don't expect to be able to ever start this engine by hand!

On the positive side, there's pushrod valve actuation instead of the belt-driven SOHC of the MD22 series - and for serious cruising yachties who are likely to visit remote locations, pushrods are more reliable than belt-driven camshafts. As the maximum engine revs are relatively low, there's really no need for OHC valve actuation, which is needed only when opting for four valves per cylinder to reduce camshaft drive complication.

As with the D2-55, the in-line fuel injection pump is driven from the camshaft and incorporates a feed pump with hand primer, while the water-separating fuel filter and canister oil filter are both mounted on the same side. The closed-circuit (heat exchanger) cooling system features an easily-accessible seawater pump and impeller, a freshwater-cooled exhaust manifold for better corrosion resistance than saltwater systems, and a thermostat for constant cylinder-head temperatures regardless of ambient or seawater temperature.

Both the alternator and 2.0kW starter motor are mounted high on the block away from bilge water - and to reduce electrolysis between the engine and saildrive, an electrical insulation is provided. The alternator output has risen from 60amp for the D2-55 to 115amp, and has voltage regulation and an electronic sensor for voltage drop compensation. Volvo Penta also offers a 220V alternator that can provide 2.2 or 3.5kW AC 50Hz, presumably achieved by mating an invertor with the standard 12V alternator.

As with the D2-55, glow plugs are fitted in the pre-combustion chambers for rapid cold starting.

BETTERPOWER, TORQUE AND FUEL EFFICIENCY
Whereas the D2-55 has a maximum crankshaft output of 55.0hp (at 1.0hp = 746W) at 3000 revs, the D2-75 develops 73.7hp at the same revs. And compared to the D2-55's maximum torque output of 135Nm at 2350 revs, the D2-75 produces 212Nm at 1800 revs - although the torque curve is fairly "peaky" with 209Nm at 1600 revs and 210 at 2000.

At 1400 revs where the D2-55 produces 120Nm, the D2-75 produces 198Nm - while at Volvo Penta's recommended cruising revs of 2500, the D2-75 produces 200Nm.

To achieve the additional power and torque over the D2-55, the trade-off is fuel efficiency at Wide Open Throttle. The D2-75 consumes 17.7lt/h compared to 12.5 for the D2-55 - a 42 per cent increase in consumption for a 34 per cent increase in power. However, throttled back to 2600 revs, where based on a standard propeller power curve the prop is absorbing 46hp, the D2-75 uses 10.3lt/h compared to 8.0lt/h for 34hp at the same revs - an efficiency improvement of five per cent.

The nearest competition to the D2-75 is the Yanmar 4JH3-TCE, which develops 73.7hp from its four-cylinder 2.0lt powerhead, but at 3800 revs. This engine has direct injection but only turbocharging without an intercooler or aftercooler, resulting in high combustion-chamber air temperatures and relatively inefficient fuel combustion - unless the engine is used in cold climates such as Tasmania, where the ambient temperature is relatively low.

And although the torque curve is relatively flat compared to the D2-75, the maximum output is only 170Nm at 2800 revs, whereas at these revs the D2-75 is still producing 189Nm. At 18lt/h at 3800 revs the Yanmar uses two per cent more fuel than the D2-75 at Wide Open Throttle - primarily a result of the inefficiencies resulting from the decision to employ straight turbocharging.

Throttled back to where the prop absorbs 46hp (3200 revs) the Yanmar uses 10.3lt/h, but the torque output is 165Nm compared to 198 for the D2-75.

LIGHTWEIGHT AND COMPACT
Complete with a mechanical MS25A eight-degree down-angle gearbox (which is better for cruising yachts because of its relative simplicity compared to Volvo Penta's hydraulic gearbox option) the D2-75 weighs 258kg - only 15kg or six per cent more than the D2-55 with this box.

Comparatively, while fitted with the 150S saildrive, the D2-75 weighs 264kg - again only six per cent heavier than the D2-55 with this drive unit.

The gear ratios for the MS25A are 2.23:1 in ahead and 2.74:1 in astern - the latter a useful reduction for backing off sandbars - while the 150S ratio is 2.19:1, still keeping prop revs below 1500 for thrust efficiency. The 150S drive is available with either three or four-blade folding props - the latter ideal for cruising yachts, as it combines low drag characteristics with more blade area for a given prop diameter, and lower vibration levels than three-bladers - although thrust efficiency may suffer slightly due to the additional blade.

Complete with an SD40-4T saildrive, the Yanmar weighs 260kg - only two per cent less, and it doesn't have the torque of the D2-75. Nor with its mechanically-controlled direct injection is it likely to comply with the 2006 exhaust emission regulations.

At 917mm long, 544mm wide and 891mm high with gearbox, the D2-55 is a compact engine - but the D2-75 is the same length, width and height, and is excellent for a diesel of this power and torque. Fitted with a mechanical KM3A down-angle box, the Yanmar measures 891 x 563 x 619mm.

As with its D2-55, Volvo Penta has again achieved the ideal combination of plenty of power and torque in a compact, fuel-efficient package - but this time one that will suit yachts to around 15m.

For more information on the D2-75, contact Graeme Avers at Eastern Engine, tel (07) 3902 5444 or email graeme.avers@volvopenta.com.au.