Diesel engine evolution

The past ten years have seen substantial changes in the diesel engine world. Three notable recent developments are electronic engine controls, unit injectors and common rail fuel injection (more about these below). Other less obvious changes are ‘invisible’ to boat owners: refinements in materials and design elements that do not affect operating and maintenance practices.

Many of the less obvious changes have occurred as a result of ‘technology forcing’ legislation, primarily in the form of ever-tightening emissions standards. When such legislation is first introduced, many in the industry argue that the new standards will be impossible to meet, but in fact as each successive deadline has approached, manufacturers have invariably succeeded in exceeding the new requirements. Some will admit off the record that the legislative pressure has been good for the industry.

Initially, most of the tightened standards were not applicable to marine engines. But because the marine marketplace is relatively small (approximately 50,000 diesel engines up to 800-hp worldwide each year, as opposed to millions in the automotive and trucking industries), many marine diesel engines have always been adapted from other applications, and in so far as the new standards applied to these applications the technology found its way onto boats.

From about 2004 onwards, marine engines have been specifically included in both international and U.S. Environmental Protection Agency (EPA) regulations, with increasingly stringent emissions requirements being phased in over the five year period from 2004 to 2009, and more now set in place for 2009 to 2014 (Tier 3).

Electronic engine controls
First generation electronic engine controls replaced the functions of a conventional mechanical governor with an engine speed sensor wired to an electronic control unit (ECU). The ECU outputs a signal to a solenoid-operated actuator that moves the fuel rack in a conventional injection pump to vary the rate of fuel injection. The rate of injection is adjusted to maintain the desired engine speed.

Over time, the inputs to the ECU have been expanded, resulting in increasingly sophisticated decision making that governs fuel rack operation. Whereas in the old days, for example, sudden acceleration would often be accompanied by a cloud of black smoke, today this almost never occurs, even on the most primitive electronically-controlled engines.

The advent of electronically-controlled unit and common rail injection brought with it a qualitative leap forward in the level of control over the fuel injection process. Whereas all previous systems operated a fuel rack that had a common effect on the injection process into all the cylinders, these new systems integrate a mass of data into decision-making processes that independently control the injection and combustion processes for each cylinder.

The latest generation ECUs are integrated with fly-by-wire throttle, transmission shifting and steering. There is a common data cable, and no mechanical link, between the cockpit displays, and the throttle, gear shift lever, and steering wheel, together with the devices they control.

Almost all ECUs operate on some variant of a protocol known as controller area network (CAN) that has come out of the automotive industry, and which is found on most modern cars (when you take your car in for servicing and the technician plugs it into a computer to check its operating history and diagnose problems, the computer is being plugged into the CAN system).

Latest-generation navigational electronics also share data via a variant of the CAN system, notably via NMEA 2000. This is bringing about an integration of navigational electronics with engine electronics. At its simplest, this may be no more than engine data being displayed on navigational screens and vice versa, but in many instances there is beginning to be functional interaction. For example, if the depth sounder shows shallow water, or the radar shows an approaching land mass, the engine may be slowed. This process of integration is certain to continue.

Electronic engine control systems result in a considerable reduction in the wiring associated with an engine. In essence, all the engine sensors have point-to-point wiring to the ECU unit, which is generally mounted on, or near, the engine, and from there all the data travels on a single twisted-pair cable to the remotely-mounted engine controls and displays, as opposed to the wires from individual sensors being routed to the gauges that display their information. The gauges themselves are invariably mounted in a serial fashion, which is to say with the single data cable looped from gauge to gauge. Increasingly, actual gauges are giving way to on-screen displays.

The net result is a powerful electronic display and control system with tremendous diagnostic, failure warning, and troubleshooting functions, and with a considerable amount of stored history, but which is, unfortunately, beyond the capability of any amateur mechanic to troubleshoot and repair.

There is an obvious need to identify critical circuits and to make them as invulnerable to disruption as possible, and, if necessary, to provide a manual override of some kind if the network bus goes down altogether. This is known as a “limp home” capability. It varies from manufacturer to manufacturer. It is one of those things that anyone considering buying a fully electronic engine should research before choosing any one system.

Regardless of how well a system is hardened against radio frequency interference-electromagnetic interference (RFI-EMI), lightning strikes, and other potentially damaging events, all systems require a minimum power supply to remain operational. If the voltage drops below a certain level (generally around 5.0 to 6.0 volts), there will be a brown-out, in which unpredictable responses may happen, followed by a complete shut-down of the network if the voltage drops lower. A boat owner would be well advised to ensure some kind of uninteruptible power supply (UPS) for the critical circuits on the network (e.g., the engine, steering, transmission shifting, and bilge pumps).

Unit injectors
Unit injection and common rail fuel systems are invariably associated with sophisticated ECUs. Unit injectors have a fuel pump that draws fuel from a tank via a primary filter, passes the fuel through a secondary filter, and then discharges it continuously into a passage, or gallery, in the cylinder head at a relatively low pressure. This gallery supplies the injectors. A pressure-regulating valve at the end of the fuel gallery holds the pressure in the system at a set point and allows surplus fuel to return to the fuel tank. Fuel flows continuously through the whole system, including the injectors, keeping them lubricated and cool.

Each injector contains its own pump, similar in many ways to a conventional in-line jerk pump. With mechanically-operated unit injectors (e.g., many Detroit Diesels), an engine-driven camshaft operates the pump. At the appropriate moment, the pump strokes and injects fuel directly into the engine. Engine speed is controlled in a similar fashion to that used to control a conventional jerk pump by mechanically adjusting the point at which the pumping element in each injector lines up with a spill-off port, terminating the injection pulse.

With electronically-controlled injectors, the fuel moved by an injector’s camshaft-driven pumping element is not necessarily injected into the combustion chamber. Instead, it flows out of a spill port until such time as an electronically-operated solenoid valve closes the spill port, at which point the pressure rises and the fuel is injected into the combustion chamber. In this way, by controlling the timing of the moment at which the solenoid valve closes and opens the spill-off port, the injection process into each cylinder can be individually managed by the engine’s computer to optimize fuel efficiency and minimize exhaust emissions. This kind of injector is known as an electronic unit injector (EUI).

On some engines (notably Caterpillars), the camshaft is eliminated and instead high pressure engine oil is used to operate the pumping element in each injector, generating injection pressures. Once again, a solenoid valve controls the injection process. By varying the oil pressure, the rate of injection can be altered along with the length of the injection pulse (which is determined by the solenoid valve). This kind of injector is known as a hydraulic electronic unit injector (HEUI).

EUI and HEUI units effectively separate the two functions of generating fuel pressure and injection timing, which are combined at the injection pump in a conventional system. In effect, the camshaft or oil pulse generates an extended pumping pulse, with the ECU determining at what point to utilize it (by triggering the solenoid valve).

Unit injection is common on larger engines, because it is possible to achieve extraordinarily high injection pressures (up to 30,000 psi/2,000 Bar) which is important in terms of fully distributing the injected fuel inside large combustion chambers (the larger the combustion chamber, and the further the fuel has to be projected, the higher the injection pressure needs to be). Unit injectors are rarely found on smaller engines (other than some Detroit Diesel two-cycle diesels).

Common rail system
A common rail system is similar in some ways to unit injection in as much as a pump draws fuel from the tank and circulates it continuously through a gallery in the cylinder head and back to the tank. However, in the common rail system the fuel is circulated at full injection pressure (often 20,000 psi/1,350 Bar or higher), eliminating the need for unit injectors. Instead, all that is required for injection is some kind of a valve to admit fuel to the cylinders at the appropriate time and in the appropriate quantities. On modern engines, this is typically a fast-acting electro-magnetic needle valve, controlled by the engine’s computer.

The common rail system once again separates the two functions of generating fuel pressure and injection timing. The fuel is stored continuously at injection pressures in an accumulator rail. At the injector, the electronic valve opens and closes to allow fuel into the cylinder, with the ECU controlling opening and closing times.

Benefits of unit injection and common rail
With a conventional injection pump, there is an ‘injection lag’ between the moment the pump generates injection pressures, and the moment of injection. This lag varies with such things as fuel temperature and viscosity, the elasticity of injection lines, and wear in the injection pump. In contrast, unit injection and common rail is ‘real time,’ enabling extraordinarily precise control.

With a conventional system, the volume of fuel in an injection line is quite small. As soon as injection commences, there is some pressure drop in the line, which reduces the effectiveness of the injection stroke. With common rail, the volume of pressurized fuel in the accumulator relative to the volume of fuel being injected is such that it eliminates this pressure drop. It is also eliminated with the direct injection of a unit injector.

With both unit injection and common rail systems, the ECU can regulate the injection process in a manner that results in a far more controlled power stroke than with conventional injection systems. There may be as many as five injection pulses per power stroke with a common rail system. The most immediately noticeable effect is to eliminate the characteristic ‘clatter’ of a diesel engine at idle. The net result is improved engine efficiency over a wide range of operating conditions, with lower exhaust pollution, less noise and vibration, and extended engine life. The downside is the dependence on sophisticated electronics.

Because of the high pressures found in unit injection and common rail systems, no amateur mechanic should ever attempt to service one of these systems. This is especially the case in a common rail system where just slackening an injector nut (as is commonly done when troubleshooting other types of injection systems) can result in a spray of diesel that has sufficient force to penetrate the skin and cause blood poisoning. The incredibly finely atomized diesel is also a fire hazard. (To minimize fire risks in the event of damage to system piping, all common rail piping is double walled with an alarm system that goes off if the inner wall gets ruptured.)

The 100-hp threshold
On engines of less than roughly 100-hp (76-kW), the principal manufacturers and suppliers of diesel engines to the marine marketplace have all found a way to comply with the Tier 2 regulations, and many of them with the Tier 3 regulations, without resorting to unit injection and common rail systems. Given the higher costs of these new technologies, we are not likely to see them in these lower power ranges any time soon. However, what is being done is to increasingly expand the role of ECU units that then operate an electrically-controlled, rather than a mechanically controlled, fuel rack.

The benefit of extending ECUs to what are essentially mechanically-controlled engines includes more sophisticated inputs into the control process, and the ability to integrate the engine data (temperature, pressure, etc.) into networked display systems (e.g., NMEA 2000). It’s a small step to fly-by-wire throttle and shifting. The downside is that the engine operation is now unecessarily dependent on sophisticated electronics. The best of both worlds may be to have the electronics, but with an emergency mechanical override (this is even possible with some unit injection systems, such as those supplied by Steyr Motors on engines destined for use in lifeboats).

Other refinements
The power-to-weight ratio of diesel engines keeps shifting upwards. In 2007, Volvo Penta released a new 11-liter engine (the D11) with a block cast from compact graphite (a form of graphite-reinforced cast iron) which is stronger than conventional cast iron, permitting thinner castings and improving the power-to-weight ratio. Volvo Penta claims the D11 sets a new power-to-weight ratio in its class. In 2009, Steyr Motors will release a new version of its monoblock six-cylinder engine which, it claims, will set a new power-to-weight ratio in this class. Volvo Penta claims its D4 is the world’s most powerful four-cylinder diesel. It’s almost certain that these new records will not stand for long.

The D11’s turbocharger features a twin-entry approach (“duopulse”) first used by Volvo Penta on the D9 engine (launched four years ago). It results in extremely rapid response and powerful acceleration, and contributes to the engine’s trademark high torque at low speeds, which is particularly useful in getting planing boats “out of the hole” and onto a plane. Other manufacturers are similarily continually refining components and innovating in ways that improve performance and efficiency.

At the drivetrain end of things, the most dramatic innovation has been the introduction of first Volvo Penta’s Inboard Performance System (IPS), followed a year or two later by Cummins MerCruiser Diesel’s competing Zeus system. These are designed for high speed planing boats with two or more engines and drive trains (some IPS boats now have four installations). The overall effect is to achieve enhanced performance over a conventional boat with smaller engines and significantly improved fuel consumption. Not surprisingly, the rate of adoption among powerboat builders has been rapid (the IPS, in particular, has been carving out a substantial market niche).

One of the more significant results of the various diesel engine developments over the past decade or so has been to substantially improve the fuel efficiency of marine diesels at light loads. This is particularly important for recreational boaters, especially those with displacement hulls (sailboats), because the engines in these boats are typically lightly loaded much of the time when under power. A side effect of this efficiency improvement has been to cut into the rationale for electric propulsion systems, in as much as this rationale is commonly based on the theoretical ability of electric propulsion to improve fuel efficiency at light loads.

It is ironic that at a time when public interest in hybrid boats is growing rapidly, the potential benefits are diminishing.

This is not to say that hybrid boats cannot be made to perform substantially more efficiently than conventional boats. It just makes the challenge that much greater, necessitating the integration of alternative energy sources (such as solar, wind and regeneration) into the energy equation, and also extending the hybrid system to include optimal management of the house loads. No system currently on the marketplace has this level of sophistication. Achieving this is the primary purpose of a European research consortium that I recently helped put together, and which is now being funded by a 2.2 million euro grant from the European Union.

Where does this leave the average boat owner? For most of those with engines smaller than 100-hp (76-kW), in spite of all the technological advances, there is actually little to no impact in the way in which the owner will interact with the engine, both in terms of operating it, and also maintaining it. This will gradually change as we see more ECUs creeping onto these engines, but fundamentally for some years yet these engines will remain conventional, mechanically-controlled diesels with conventional fuel injection systems.

The big changes have come with larger engines, and are filtering down towards the 100-hp level. Here, sophisticated electronics have taken over. The boat owner can still carry out oil and filter changes, and other routine maintenance, but beyond that the engine is off-limits. Faced with problems, you need to call a trained professional with the necessary computerized tools. For some, this is a disconcerting development, but if the automotive world is anything to go by, it should bring with it ever more reliable and long-lived engines.

Legislation for cleaner diesels

The first major standards regulating marine diesel engines were adopted in 1973 as Annex VI to the International Convention on Prevention of Pollution from Ships. This was subsequently modified by the Protocol of 1978, known as MARPOL 73/78. These standards formed the basis of the U.S. Environmental Protection Agency’s (EPA) Tier 1 standards, developed as a result of the passage of the Clean Air Act in 1990, but only applied to automobiles and light duty trucks at that time. The Tier 1 standards were fully implemented in the automotive field by 1997.

The Annex VI standards themselves only apply to engines of more than 170-hp (130-kW) installed on vessels constructed after January 2000, and were not enforceable until May 2005. Beginning soon after 1997, the Tier 1 standards were extended to cover (i) marine engines of more than 30 liters per cylinder (that’s a huge engine), with full implementation by February 2003, and (ii) marine engines of more than 2.5 liters per cylinder (that’s still a big engine), with full implementation by January 2004.

Meantime, the EPA developed much tougher standards for automobiles, trucks, lawn mowers, and numerous other applications. These are known as the Tier 2 standards. These now apply to all engines (including marine) with phased implementation from 2004 to 2009, depending on engine size (the smaller engines were phased in first).

In March 2008, the EPA introduced Tier 3 and Tier 4 emissions standards for marine diesel engines. The Tier 3 standards apply to all marine engines. The Tier 4 to engines greater than 800-hp (600-kW). The Tier 3 standards will be phased in between 2009 and 2014, once again with the smaller engines first.

The Tier 2 standards progressively enforced radical reductions in certain exhaust emissions, which will be further toughened by Tier 3. In automotive and trucking applications, these reductions limit some emissions on a per mile basis, irrespective of engine size (i.e. a large engine is not permitted to produce any more emissions per mile than a small engine). As a result, the larger an engine, the bigger the challenge in complying. In the marine world, emissions are measured in terms of grams per kilowatt-hour, with the bigger engines required to produce less of some emissions per kilowatt-hour than the smaller.

Either way, given the current state of diesel engine technology, large engines (greater than 2.5 liters per cylinder) are only capable of meeting the Tier 2 and 3 standards by using electronic engine controls with common rail or unit injection, hence the rapid spread of these technologies on high powered engines. Smaller marine diesel engines can meet the Tier 2 standards, and many of them the Tier 3 standards, with more traditional technology, hence the survival for some years to come of these engines in the marine world, although their days are almost certainly ultimately numbered.

The Tier 4 standards are particularly controversial, because catalytic aftertreatment of the exhaust is required to meet them. This, in turn, requires low-sulfur fuel, which is already mandated for shoreside use. The EPA has set a limit of 15 ppm for marine diesel by 2012. However, boats venturing overseas are likely to encounter high-sulfur fuels, which will mess up the catalytic converters. A subsidiary problem is salt water damage to the converters. Salt water is, of course, currently present in all wet exhaust systems. Although in its present form, Tier 4 only applies to really large engines. At various times, the EPA has proposed applying it to all marine diesel engines.

It is worth noting that none of these emissions standards address carbon dioxide emissions. In fact, catalytic converters increase carbon dioxide emissions.

See www.dieselnet.com/standards/us/marine.php for an excellent summary of emissions regulations.

Nigel Calder

By Ocean Navigator