Talking torque: Twinning All the Way

Light and throttle responsive...

Twin cams and common rail fuel injection are all the rage at the moment. Nannidiesel's Toyota-based range of marine diesels are being snapped up for re-powers in planing cruisers.

But what about less powerful engines? 

Naturally aspirated indirect injection diesels have much higher compression ratios (21:1–23:1) than comparable direct injection engines (15:1–17:1) and tend to suffer air starvation, which contributes to higher fuel consumption. But adding a low boost turbocharger eliminates this issue without stressing the engine's bottom end, as inter-cooling could. Traditionally adding a turbocharger adds around 12 per cent extra horsepower, but around 30 per cent more torque at the same revs, and actually makes the engine more fuel efficient relative to output because it breathes better. 

In my opinion, if boaters want inter-cooling, they should consider direct injection diesels instead, which benefit from cooler, denser air in combustion chambers and develop way better torque and peak output for displacement than straight-turbo direct injection engines.

THE NITTY GRITTY

From Nannidiesel's tech specs, the twin cams in the N4.65 appear to be chain driven, which, in my opinion is better for an inboard marine engine than belt-driven camshafts. The drive chain is sealed and runs in oil whereas a belt can be damaged by damp salty air. Well-designed chain drives normally last the life of the engine too, unlike belt drives. I would presume having a Kubota base means the chain is a duplex unit, which is far less likely to stretch than the single chain drive used by some car makers.

Having twin cams eliminates the friction and drag caused by push rod valve actuation and allows the engine to rev more freely. It also allows for larger intake and exhaust porting in cross flow cylinder heads, which is limited when push rods are used — which is why push rod valve actuation isn't used in twin cam engines. Replacing cylinder gaskets is slightly less finicky, too, once the camshafts have been removed.

In addition to the DOHC valve layout and 16 valves, the N4.65 has common rail injection, which helps the engine meet all the current Euro and US exhaust emission regulations, through its higher injector pressures compared to conventional in-line systems. This is assisted by Kubota's ‘Tri-Vortex’ combustion chamber design, which, in conjunction with the pre-combustion chambers into which the fuel is sprayed, creates much finer fuel droplets than traditional injection systems. As such, there is a better air/fuel burn with fewer particulate emissions, though no diesel engine can completely eliminate these.

Fortunately the common rail appears to be mechanical and not electronic, which makes for a system that's easier to service when cruising in regions that have limited servicing facilities — though it does prevent straightforward installation of fuel flow and range to empty instrumentation. Nannidiesel's C4 instrument panel includes an analogue tachometer with digital hour meter, plus gauges for low oil pressure and high coolant temperature in the standard heat exchanger cooling (keel cooling with a dry exhaust is optional). The heat exchanger system has the usual positive displacement sea (raw) water and centrifugal freshwater circulating pumps with an expansion tank.

The standard voltage regulated alternator produces up to 120A and is mounted well up on the engine away from any possible bilge water. Sensibly, Nannidiesel presumes that yachties ain't fools and doesn't fit a cover over the v-belt that drives the alternator and freshwater pump. Decompression levers and manual starting aren't fitted, saving owners the embarrassment of not being able to start the engine in the event of a flat starter battery should guests be aboard. With the 23:1 compression ratio, it would be doubtful the engine could be kept spinning fast enough to fire up even if manual decompression levers were fitted and then closed, unlike the old direct injection engines. 

Like all Kubota-based Nannidiesel engines, the N4.65 has a cast iron cylinder block and head for longevity. Yet with the standard mechanical Italian-made Technodrive TMC260 gear box, the N4.65 weighs only 241kg dry. Complete with this box, the N4.65 is 926mm long, 515mm wide and 632mm high, making it smaller than the naturally aspirated 49.3bhp Nannidiesel N4.50 and only 11kg heavier. This 2197cc engine has the same cylinder bore dimension as the N4.65 but a much shorter piston stroke. 

The N4.65 is available with a comprehensive range of mechanical and hydraulic gear boxes, plus the SP60 sail drive, which has the same 2.15:1 gear reduction as the TMC260 box. At WOT the prop revs are a low 1260 which enables course pitch and thrust efficient props to be fitted, as the rule of thumb for displacement hulls is that prop speed should never exceed 1600rpm.

THE WRAP

Frankly I was surprised that Kubota made an engine such as the N4.65. The less powerful Kubota-based engines are very conventional in their design and engineering. But what I do like is that Kubota didn't go down the track of stressing its twin cam engines with a light duty rating and way higher revs. Kubota has shown it is possible to combine free-revving technology with engine longevity, whereas the current diesel engine design trend is to make diesels that are stressed to the max, even in trawler yachts, and fit excessive power in displacement hulls that wastes fuel.

Sure, the higher outputs from small displacement engines may look impressive and open up more accommodation space, but being an old fart raised on lazy, long life engines I prefer diesels that are designed and engineered to provide their owners with longevity, while not breaking the bank if mechanical issues need fixing. 

The N4.65 is way under-stressed compared to the two-litre N4.60 model it replaced, which developed comparable torque but 59.2bhp at 2800rpm while using 11 per cent less fuel at WOT. Complying with current exhaust emission requirements has reduced output and raised fuel flow, an issue I've noticed with other indirect injected and mechanically controlled diesels over the past few years, such as the Nannidiesel N4.50. 

For more on the N4.65 call Nannidiesel Australia on (02) 9319 5222 or visit nannidiesel.com.au. 

SPECS

Engine Nannidiesel N4.65

Engine type Turbo-charged four-cylinder indirect injection diesel

Max output 58.2bhp/59.0mhp at 2700rpm*

Max torque 168NM at 1800rpm

Piston displacement 2434cc

Bore x stroke 87 x 102.4mm

Dry weight with box 241kg

*Brake horsepower/metric horsepower or PS