Cat TRB-Cat C7

European and US diesel manufacturers are currently raving about the benefits of high pressure common rail fuel injection. Even manufacturers of larger marine diesels such as MAN and MTU are switching their engines over to this system.

Common rail functions by forcing fuel along a tube to individual electronically-controlled injectors, usually at around 1600bar or 22,400psi to optimise fuel spray into the combustion chambers. This system is much more efficient than mechanical injection systems, which of course suffer from fixed injection timing which is either too advanced or retarded depending on the engine revs.

A DIFFERENT APPROACH
With its compact C7 Cat Marine has taken a very different approach and has used the same system as in its much more powerful C-series engines.

Known as HEUI or Hydraulically-actuated Electronically-controlled Unit Injector fuel system, the system provides infinite control of injection pressure regardless of engine revs as really only unit injectors can. Cat Marine states that HEUI's flexibility creates many performance advantages such as improved responsiveness, greater fuel economy and reduced exhaust emissions.

A portion of the engine's lubricating oil is used to power the injectors instead of mechanical force from a camshaft lobe and rocker arm. The high pressure oil pump uses this lube oil flow to generate the "actuation oil pressure" of between 62–250bar or 870–3500psi to power the injectors. The HEUI injectors then use this pressurised oil to produce injector nozzle pressures of around seven times greater than the actuation pressure, resulting in a massive 1750bar or 24,500psi and currently one of the world's highest injector nozzle pressures available.

To put this pressure in perspective, unit injector diesels such as Lister's popular SR series of naturally aspirated marine engines have nozzle pressures of 143bar or 2000psi.

As with the C9, the C7 engine's electronic control system continually monitors key engine functions such as barometric pressure and ambient temperature, coolant and lube oil temperature and engine load and adjusts the injector spray timing accordingly. For example, when the throttle is opened rapidly the management system will progressively supply fuel to reduce black smoke emissions. And should the throttle be opened when the engine is cold the revs will be increased in stages until the engine has reached normal operating temperature.

COMPETITIVE MARKET
Available in the US since May last year and locally late in the year, the C7 has had to compete directly with the Iveco Cursor 500, Volvo Penta TAMD75P-EDC and Yanmar 6CXM-GTE2 since its launch, engines which in twin installations suit hulls such as the Blackwatch 40. Cat Marine has also pitched the C7 against the Cummins 480C-E but in my opinion really the C9 competes with this engine.

A straight six and a development of the 3126B, the C7 develops a maximum crankshaft output of 454hp (where 1hp = 746 watts) at 2800 revs when rated "E" or high performance for planing-hulled vessels. This rating allows for half an hour of Wide Open Throttle operation in every six and between 250–1000 hours of operation per year, whereas one of the competitors is rated to only 300 hours per year. Another aspect is that Cat Marine rates its engines on distillate with a density of 839 grams per litre, the same as Aussie fuels, whereas European and Japanese manufacturers rate on distillate of 860 grams. When operated on our poor quality fuel the maximum outputs must be substantially reduced.

Displacing 7.24 litres, the turbocharged and seawater-aftercooled C7 has the smallest displacement of engines in its power range. The Cursor 500 displaces 7.8lt and develops 493hp at 2600rpm, the TAMD75P-EDC 7.28lt and 473hp at 2600rpm and the 6CXM-GTE2 7.41lt and 493hp at 2900rpm.

Unfortunately the TAMD75P-EDC is heavy for its output (1045kg bobtail) so the Iveco and Yanmar engines are the real competition. The Cursor 500 has a bobtail weight of 910kg and the 6CXM-GTE2 825, while the C7 weighs only 798kg.

TORQUE TO WEIGHT
As with most electronically-managed diesels the C7 has a relatively flat torque curve, with a peak output of 1420 Newton Metres from 1800–2200 revs. In comparison the Iveco produces 1510Nm from 1800–2000 and the Yanmar 1500Nm from 2000–2200, making the C7 the least torquey in this output range.

On the Aussie market the Yanmar is the main competition to the C7, so how does this mechanically-injected engine compare with the fully-electronic Cat, which has three valves per cylinder compared to four for the Yanmar?

Both Cat Marine and Yanmar provide average fuel flow curves according to revs based on a standard prop power curve - see the table for a detailed comparison.

At 1442mm long including the turbocharger the C7 is 62mm shorter than the Yanmar; at 920mm wide it's 96mm wider; and at 917mm high it's 12mm higher. These measurements include the safety covers for the high-mounted alternators, which along with the starter motors are 12 volt systems.

At a time when all the other manufacturers have or are in the process of going down the common rail track it's refreshing to see a company that retains its individual approach to fuel injection. As one of its main European competitors told me recently, unit injectors are still the answer for engines 12 litres and above because of the more accurate control of injection timing possible. But instead of mixing the systems Cat Marine has retained commonality across its marine diesels, in my opinion a logical direction to take.

For more details on the Cat Marine C7 engine range, contact Lindsay Arndt at Energy Power Systems, tel (07) 3722 1476, email lindsay.arndt@energypower.com.au