The new-generation electronic diesels - Soundings Online

The new-generation electronic diesels

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Pollution regulations are forcing the retirement of the smoke-belching, noisy but eminently reliable, hard-to-kill-even-if-you-tried mechanical diesel. Its replacement is a marvel of the microprocessor age where combustion is precisely controlled by a computer.

Pollution regulations are forcing the retirement of the smoke-belching, noisy but eminently reliable, hard-to-kill-even-if-you-tried mechanical diesel. Its replacement is a marvel of the microprocessor age where combustion is precisely controlled by a computer.

The new electronic marine diesels fall into three categories: electronically governed mechanical injection; hydraulically actuated electronically controlled unit injection, or HEUI (Caterpillar); and common rail fuel injection system, or CRS (Cummins MerCruiser, Volvo Penta, John Deere, Lugger, MTU). The electronic control of combustion, especially when combined with a HEUI or CRS fuel system, allows these engines to run cleanly and efficiently over a wide range of rpm and loads. A CRS system is available on engines as small as the 106-hp Volvo Penta D3.

This new generation of diesels solves several problems inherent in the older models. Chief among them is this: A mechanical engine capable of powering a boat at semidisplacement and planing speeds cannot be operated continuously at displacement speeds for long-range cruising or trolling without damaging the engine and reducing its life. Among other undesirable effects, low-load operation causes carbon buildup throughout a diesel engine, thus the recommendation from mechanics to “clean” the engine by running it periodically at full-rated load.

For a cruising trawler with a semidisplacement hull, like a Grand Banks Heritage or Nordic Tug, having a high-power engine that performed well at semidisplacement (high) speeds meant poor performance at displacement speed. A comparison between the propeller demand curves for a Caterpillar 3126 mechanical engine and a 3126B HEUI engine (now the C7) demonstrates how the HEUI and CRS, which are different technical solutions that are functionally similar, solve the problem.

The microprocessor-based electronic control and HEUI fuel injection on the 3126B eliminates the cam-driven fuel injection pump, thereby severing the mechanical linkage that restricted fuel injection timing flexibility. The table below shows the fuel consumption from the propeller demand curves at various rpm for both engines. Notice how much more efficient the electronic engine is at the low end of the rpm range. For example, at 1,200 rpm the mechanical engine is burning 6 gph while the electronic HEUI engine burns only 2.2 gph.

Both HEUI and CRS have the added important feature over electronically governed mechanical systems of injection pressure being independent of engine speed and quantity of fuel injected. Therefore, very high injection pressure can be achieved at low loads for better combustion (see graph below). Additionally, since injection timing and duration are independent of cam lobe profile with HEUI and CRS, these events can be configured with a level of precision not possible in mechanical systems to match prevailing conditions, including multiple injections.

Are HEUI and CRS going to be troublesome on boats? Bosch, the leading CRS manufacturer, has produced more than 100 million common-rail injectors fitted in more than 23 million passenger cars, and Caterpillar has produced hundreds of thousands of HEUI engines. This is proven, reliable technology that vastly improves combustion at varying loads, meeting the most stringent pollution requirements. Most importantly it adds the flexibility, within reason, to have an efficient displacement long-range cruiser that also goes fast.

It’s important to note that just because an engine is marketed as “electronic” doesn’t mean it uses a CRS or HEUI system. The Cummins MerCruiser Quantum Series (QSB5.9, QSC8.3, QSL9); Volvo Penta D3, D4 and D6; John Deere PowerTech 6081AFM; Lugger 1066H; and MTU Series 2000CR and Series 4000 are CRS diesels. The Caterpillar 3126B, C7, C9, C30, C32 and 3412E are HEUI diesels, and Cat says it will introduce a CRS engine that will be called the C6.6.

The real world

Maramor, my Grand Banks 42 with a semidisplacement hull, has a single Cat 3126B rated at 420 bhp. She has been run for four seasons (950 hours) at 1,800 rpm (28 percent load), which gives a cruising speed of 9 knots and a fuel burn of 6 gph. She is never run at rated rpm to “clean” the engine — a practice I believe is wasteful with an engine that isn’t being overfueled, due to the ability of the HEUI fuel system to precisely meter the fuel at all loads.

At every oil change the lube oil is analyzed and close attention is paid to her operating parameters, so actual operation has proven that the electronic diesel adds the flexibility of continuous low-load operation with high-power engines. But how low can you go without damaging these engines? When Maramor was delivered with her HEUI engine, Caterpillar told me 30 percent (126 bhp) out of concern that there be sufficient piston ring sealing pressure. At the Miami International Boat Show in February I posed this question to engineers from all the manufacturers, including Caterpillar, and the consensus was that the safe limit with HEUI and CRS is actually much lower, more in the range of 20 percent continuous low-load operation. One CRS manufacturer told me 10 to 20 percent should be no problem. In Maramor’s case 20 percent is 84 bhp, perfect for efficient long-range cruising.

As a rough rule of thumb a displacement hull requires 1 hp per 500 pounds of fully loaded displacement. Maramor displaces 40,000 pounds with full fuel and water. Using this rule of thumb, she requires about 80 hp to cruise at a displacement speed of about 8 knots. The 3126B is offered in various ratings from 250 bhp to 450 bhp, and the rating can be changed by altering the programming in the electronic control module and changing the piston crowns, injectors and turbocharger. A lower power rating, such as the 315-bhp version in the table, would make a lot of sense for a long-range coastal cruiser like Maramor, since the power required for displacement speed would be obtained at a lower specific fuel consumption and at a lower rpm for enhanced propulsive efficiency.

I spoke with two owners of new Nordic Tug 37 trawlers, and both report that their single Cummins MerCruiser QSB5.9 380 bhp CRS engines also deliver the flexibility of efficient continuous low-load performance for long-range cruising at displacement speed, and permit high-speed operation at semidisplacement speeds.

How it works

The heart of the mechanical diesel is the cam-driven, reciprocating, positive-displacement fuel injection pump, and the heart of the fuel injection pump is the precision-machined barrel and plunger. Incredibly, the clearance between the barrel and the plunger is just a few microns. Nigel Calder in “Marine Diesel Engines” observes that a 4-cylinder 4-cycle engine running at 3,000 rpm and burning 2 gallons of diesel per hour on every compression stroke injects only 0.0000055 gallons of fuel (5.5 millionths of a gallon) timed to an accuracy of better than 0.00006 seconds. A miracle of mechanical precision, indeed.

The plungers are machined in such a way that when rotated by the fuel rack linked to the mechanical governor, they meter the fuel to accommodate changing loads, like when the propeller comes out of the water in a seaway. However, it is the machined profile of the cam lobe that determines injection timing, and this shape is unchangeable. As such, the shape of the lobe is optimal only for the chosen design point, and timing can’t be changed for all other operating conditions without adding substantial mechanical complexity. The result of less-than-optimum timing when operating above or below the design point is pollution, inefficiency and increased maintenance. Injection rate is controlled by the mechanical governor, influenced only by engine speed and load, and injection pressure is rpm dependent.

The CRS separates the functions of pressure generation and injection of the fuel by storing it under high pressure in an accumulator — i.e. the common rail — which is essentially just a pipe supplying high-pressure fuel to individual electronically controlled injectors. Thus, injection pressure is independent of engine speed and quantity of fuel injected. High pressure delivers the fuel to the combustion chamber in a fine mist for complete combustion. At a precisely defined instant the electronic control unit, or ECU, transmits an activation signal to the injector solenoid to initiate fuel delivery. The injected fuel quantity is a function of the injector opening time, duration and system pressure. The ECU controls all injection parameters, such as the pressure in the common rail, the timing and duration of injection, and the matching of the mass of combustion air available from the turbocharger to the fuel injected. The result is reduced exhaust emissions, lower engine noise and vastly improved low-load performance.

The heart of the CRS is the injector, and those found on marine engines are the solenoid-type (above). An in-depth explanation of how the common rail injector works is beyond the scope of this article, but I can explain it in general terms. (Since major marine engine manufacturers like Cummins MerCruiser and Volvo Penta fit their engines with the Bosch CRS, the drawings used here are Bosch CRS components used in automotive engines, which are functionally the same as those fitted on marine engines.)

The outlet restrictor (8) is opened when the solenoid is activated by the ECU. On the drawing there doesn’t appear to be space for the upward movement of the solenoid armature (3) because the movement is just a few microns. The inlet restrictor (10) prevents complete pressure compensation with the result that hydraulic force on the nozzle-needle pressure shoulder (6) opens the nozzle. The movement of the valve plunger is by hydraulic force because the forces required cannot be generated by the solenoid. The Cummins Quantum engines have two injections at low load, a pilot injection and a main injection.

In the latest European common rail automotive diesels, the solenoid is dispensed with. Instead, several hundred thin piezo crystal wafers expand rapidly when an electric field is applied to them, generating a large force that is hydraulically transferred directly to the nozzle needle for a lighter, more compact and precise injector. The piezo type is fitted in high-end automotive diesels but will surely find its way to marine engines in due course.

With the piezo injector you probably will see several injections per injection cycle — for example, two pilot injections, one main injection and two post injections with smaller quantities and shortened intervals between injections.

Fuel issues

Now a note about fuel quality. CRS and HEUI fuel engines require clean, water-free diesel fuel. Cummins MerCruiser mandates a primary fuel/water separator with a 10-micron rating for its QSB5.9, QSC and QSL CRS engines. Such a fine micron rating in the primary can cause excessive fuel inlet restriction problems with poor-quality fuel, which can be overcome if the capacity of the primary filter is increased to compensate. Cummins requires a 3-micron secondary filter. No manufacturer recommends a 2-micron primary, and I recommend against that practice.

An often overlooked factor is that an adequate fuel supply pressure, best achieved through staged filtration, is essential to prevent cavitation or aeration damage to the injectors, pump and other fuel system components caused by fuel starvation. In addition, engine performance may be affected by fuel inlet restriction above the manufacturer’s limits. The best practice is to follow the engine manufacturer’s recommendation with respect to the micron rating of the primary filter (e.g., Racor 30- or 10-micron elements), and use as large a filter element as possible to prevent fuel starvation.