Under-stressed

The TAMD31S is a de-rated version of its TAMD31P inboard and KAD32P sterndrive counterparts, developing 74kW or 99.2hp (at 746W = 1.0hp) at the crankshaft at only 3000rpm from its 2.4lt turbo-intercooled direct injection powerhead.

In comparison, the same-displacement TAMD31P develops 147.5hp at 3900rpm, while the supercharger-equipped KAD32P develops 167.6hp at 3800rpm, which means this new Volvo is a very under-stressed engine. Fitted with the maximum reduction gear ratio, it would be a good re-power unit for older displacement cruisers to around 12m that have naturally aspirated diesels such as the 3.9lt Perkins 4.236 or the Ford Lehman 80hp four cylinder unit.

The 4.236 developed 76 crankshaft horsepower at 2250rpm and produced 250Nm of torque at 1600rpm, but with a 2:1 reduction gearbox weighed 519kg. But the TAMD31S weighs 370kg with a hydraulic HS25A box, and at 2250 engine revs with the 2.71:1 ratio, its prop revs would actually be lower than the Perkins.

A four cylinder version of the popular 3.6lt straight six TAMD41H to KAMD300 EDC models, the TAMD31S has an unusual (for a diesel) oversquare powerhead with a bore of 92mm and stroke of 90, significantly reducing piston speed and ring wear even at Wide Open Throttle (WOT). At 17.5:1 the compression ratio is relatively high for a direct injection turbo-intercooled diesel.

The TAMD31S has no direct competition. The nearest Yanmar model, the 1.8lt turbo-intercooled direct injection 4JH2-UT(B)E develops 99.2hp but at 3600rpm, while the 2.8lt turbo-intercooled direct injection MerCruiser D2.8L D-Tronic with electronic fuel management develops 165hp at 3700rpm. Complete with a single speed Volvo MS25 gearbox, the TAMD31S weighs 364kg, whereas with a similar box the Yanmar weighs 244kg and the MerCruiser 461.

As with its new D2-55 counterpart reviewed recently, the TAMD31S has the usual Volvo features of a cast iron cylinder block and head and replaceable valve seats, but also replaceable wet cylinder liners for quicker overhauls. The oil-cooled pistons have two compression rings and one oil scraper ring, while the crankshaft is supported by five bearing sets. For reliability, valve actuation is by pushrods from a high-mounted camshaft, which also drives the fuel injection pump.

The fuel system utilises a rotary injection pump with a reliable mechanical governor, an exhaust smoke limiting device, a water-separating fuel filter and a hand primer incorporated in the feed pump. The fuel supply is cut-off electrically.

Closed circuit (heat exchanger) cooling is standard with thermostatic control, which maintains the coolant temperature at around 94°C for efficient fuel combustion and a total coolant volume of 13lt. The expansion or coolant overflow tank is transparent for instant sighting of the coolant level.

A gear-driven seawater pump with an easily-replaceable impeller is fitted, the turbocharger is freshwater cooled and the exhaust manifold has seawater cooling with a stainless steel insert in the exhaust elbow.

A 14V 60amp alternator with voltage regulation and dual-battery charging capability is fitted, along with a 3.0kW starter motor. But unlike the D2-55 the fuel and oil filters are located on opposite sides of the block and the starter motor is mounted much lower.

A choice of three gearboxes is provided, the hydraulic unit having an 8° down angle at the output flange and ratios from 2.29:1 to 2.71:1, both for clockwise or counter-clockwise rotation. A trolling valve kit is available to reduce boat speed when manoeuvring in marinas.

One mechanical box has the same output down angle, while the other has a straight output. Although handed boxes are available, the ratios are not the same and range from 2.10:1 to 2.74:1. However, for cruising yachts and long range motorsailers where reliability and prop-freewheeling capability are essential, I would still choose a mechanical box over a hydraulic unit.

But the most impressive feature of the new motor is its flat torque curve. For example, although the maximum output is 285Nm at 1800rpm, at 1400rpm it still produces 252Nm (the same as the Perkins 4.236 at 1600), while at my recommended maximum continuous cruising revs of 2500 the torque is 261Nm. Even at 2800rpm the figure is 250Nm and only above this does the torque fall off to 236 at 3000rpm.

The Yanmar develops a maximum torque output of 232Nm at 2900rpm with the output dropping off significantly either side of these revs.

For a diesel developing the torque it does, the TAMD31S is a fuel efficient engine. Based on a standard propeller power curve, at 1400rpm, where the prop is absorbing 9.7hp, the motor consumes just 3.0lt/h. At 1800rpm and 20.6hp the consumption is 5.4lt/h, while at 2500 and 55.0hp it is 12.3lt/h, where the turbo boost is at its maximum of around 18psi. At WOT the TAMD31S uses 20.5lt/h, still very good for an engine of this output.

An example of this efficiency is at 2750rpm, where on the prop power curve the TAMD31S absorbs 74hp. The fuel consumption is 16.0lt/h compared to 26.6 for the same output from the Yanmar at 3300rpm. And at 2120rpm, where the Volvo absorbs 35hp, it uses 7.6lt/h compared to 13.3 for the Yanmar at the same output but at 2600rpm.

Fitted with a mechanical down angle MS25A box, the TAMD31S measures 1041mm long, 663mm wide and 727mm high, whereas the Yanmar is about 100mm shorter and lower, as would be expected from an engine having only 75% of the Volvo's piston displacement. But at 1244 x 762 x 766mm the MerCruiser D2.8L D-Tronic is a significantly bulkier engine.

According to Volvo, the TAMD31S will operate satisfactorily with a maximum forward inclination of 5°, an intermittent rearwards tilt of 15° and an intermittent side tilt of 20°, so the motor can be used safely while motorsailing.

Volvo supplies full instrumentation including a key-operated starter switch, coolant temperature and oil pressure gauges, tachometer and hourmeter, voltmeter, instrument lighting and an alarm system with test function for coolant temperature, oil pressure and the charging circuit.

For more information on the TAMD31S, contact Eastern Engine, tel (07) 3899 1622.