1997. Direct VS indirect injection outboards.

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Clean energy: injection compared

From Motor Boats 1997, no. 2, March, pp. 56-58.

Regulations call for more environmentally friendly engines. Some manufacturers have invested in four-strokes, while others have preferred to adopt new electronic fuel injection systems for two-strokes. Let’s see which ones.

In order to operate, a gasoline engine theoretically needs to “breathe in” a gaseous mixture consisting of 14.7 parts air and one part fuel. “Theoretically” in that the true ratio between the air inhaled and the amount of gasoline required to ensure good combustion varies both as the operating conditions of the engine itself change and as a function of the environment in which it operates. In cold starts, for example, to facilitate combustion, the amount of gasoline in the air is increased over the amount theoretically required via the choke. The same need arises during acceleration phases, when all the power is needed immediately. Conversely, the air-fuel ratio should go down (so there is less gasoline than would be needed) when the engine is running at constant rpm, a need obviously induced by the attempt to minimize fuel consumption. Regardless of these and other considerations, however, the fact remains that the engine must constantly breathe a gaseous mixture that is homogeneous and balanced in its components, two characteristics that are ensured by the presence of the carburetor.

The function of the carburetor

These are nothing more than a variable-section duct within which all the air that the engine can draw in flows. When the fluid arrives near the change in cross-section (Venturi) it is abruptly accelerated, a phenomenon that in turn induces a great loss of pressure in the air. It is precisely this loss that “sucks up” the gasoline present in a reservoir bringing it through an orifice called a “jet” that atomizes it. This produces the intended vapor of air and gasoline, which in the combustion chamber is vigorously stirred and made to burn. In words there is nothing complex but in reality things are not as simple, especially when you consider that fluids in addition to running all over the place also tend to do what they want and rarely what the designers want. Moreover, all the phenomena that are triggered within the various ducts cannot be controlled except by playing with the sections and geometric shapes of the ducts themselves or through the insertion of auxiliary devices that, intervening at the appropriate time, improve the overall functionality of the carburetor but, at the same time, also make it complicated in construction and delicate in operation. It follows that not even the most advanced carburetor is perfect and capable of responding as effectively to all the power demands of the rider. If, to return to the previous example, one wants to prioritize cold-running functionality, one will hardly be able to optimize fuel consumption at cruising speed, while if one demands maximum performance at high revs one will have to accept a more or less pronounced loss of functionality at low revs. All these compromises in the automotive field in recent years have been overcome thanks to the advent of electronic injections, fuel systems that are “intelligent” and therefore capable of responding to the functional needs of the engine or to the demands of the driver on the basis of much broader work plans than a carburetor could guarantee. Now these technologies are increasingly being used in the marine field as well.

The electronic control

Fuel injection systems base their operation on the presence of a pump that puts gasoline under pressure and a valve called an “injector,” which, opening and closing at the right time, lets through its body the amount of fuel necessary for the engine to run properly. The gasoline, once past the valve, then goes to mix with the air. The amount of fuel required by the various working situations an engine may encounter are stored in a computer whose “decisions” are driven by means of a large set of sensors that keep track of both the functional state of the engine and its operating conditions. The result is a rather complex and delicate system, but one that once fine-tuned is unrivaled in terms of functionality, especially since the market has made available ever faster and more refined microprocessors at ever lower costs. In parallel with the use of computers, component performance has also been improved to the point where it has become possible to switch from indirect to direct injection, the latest frontier for powertrain engineers.

Direct or indirect injection?

In two-stroke engines, it is well known that the intake phase is always quite critical because the piston, moving toward its bottom dead center, opens both the exhaust and intake ports at the same time. This causes, despite the efforts of technicians and engineers, some of the fresh gases to leak into the exhaust system even before the compression phase begins. The only way to prevent this situation from being triggered would be to inject fuel directly into the cylinder before compression begins and then after the piston has closed the exhaust port. At that point, however, the pressures inside the cylinder begin to be high (so it would take fairly powerful pumps, which cost money and weight), and in addition, the gasoline does not have enough time to mix evenly with the air before the spark plug triggers combustion. Hence the choice to inject the fuel into the crankcase or intake manifolds so as to work with lower pressures and to give the gasoline time to form combustion vapors. Injection systems made in this way are thus a great step forward compared to the “old” carburetor but do not guarantee the absence of unburned hydrocarbons in the exhaust, i.e., simply put, to have leaks. Direct injections, on the other hand, overcome this problem as well, proposing themselves at the top in terms of technology and performance.

The Yamaha system

Yamaha is an industry that knows a lot when it comes to injections, if only because of the experience accumulated through years of competition on two and four wheels. Now it has decided to come to market with a new generation of two-stroke marine engines that in mechanical layout and performance closely resemble their carburetor-powered cousins but provide the user with significantly lower fuel consumption. Yamaha’s system is of the indirect type and relies for carburetion control on a highly refined oxygen sensor capable of “reading” the quality of the gaseous mixture directly in the combustion chamber. Other sensors keep an eye on the air pressure and temperature, the temperature of the cooling water, and the angular position of the throttle, the parameter by which the pilot informs the computer about his wishes: if the angle increases it means that the yachtsman is accelerating, if it remains constant he is cruising while if it decreases it means that one is approaching docking. There is, of course, no shortage of phase and rpm sensors to ensure that injection occurs at the right time. The system is driven by two pumps, one for gasoline and one for lubricating oil, which cause the respective liquids to flow into a single tank where mixing takes place. From here the mixture then passes into the main pump, which sends it to the six injectors, one for each cylinder. It should be noted that the system also governs the ignition, thus integrating all the vital functions of the engine into a single logic, and is equipped with self-diagnostics: in practice, this means that in the event of a failure, it is the computer itself that analyzes the system, identifying the fault and reporting it to the mechanics. For now, Yamaha’s system is fitted only to the new 250-horsepower V6 called the 250B E.F.I. (Electronic Fuel Injection), whose main mechanical feature is inherent in the narrow V architecture of the cylinders (76°), but it will soon be adopted in the remaining engine range.

The WTO technique

OMC had been working for years on the injection system for two-stroke engines called “Orbital,” the same system that had attracted the interests of many automakers including Fiat and Ford. It was precisely because of this experience that OMC was able to acquire the technical knowledge that today forms the basis of the Ficht FCC (Ficht Fuel Injection) system, where Ficht is the name of the German company that first developed the system and is now 51 percent owned by OMC.

First of all, it should be mentioned that this system is direct injection, a choice that places the company at the top among all manufacturers of two- and four-stroke engines. Secondly, it should be pointed out that the Ficht does not mix lubricating oil with gasoline in the traditional percentages but only to a very limited extent. In practice, the lubrication system is separate and mixing between the two fluids occurs only at the vapor level: just enough to keep the injectors clean. This, in fact, results in better combustion (let us not forget that oil, although necessary to ensure the life of the engine, when mixed with gasoline still becomes its own pollutant) and absolutely negligible oil consumption when compared with that offered by engines of equal power. Then, as we mentioned earlier, there is the matter of direct injection, which is achieved thanks to the presence of a high-frequency injector combined with a high-pressure electric pump: not a single drop of gasoline can therefore “escape” from the exhaust port, to the benefit of fuel consumption and performance. Again, there is no lack of a network of sensors monitoring the powertrain, nor is the ECU that controls the fuel supply divorced from the ignition system. OMC for now has put its system on a 150-horsepower 6V engine called the “Ficht,” the first examples of which are currently being tested at the most qualified dealers: fuel consumption is claimed to be 35 percent lower than that offered by the conventional carburetor-powered 6V and a reduction in unburned hydrocarbons (highly polluting) of 80 percent.

The Selva Marine project

Selva di Tirano is the only Italian company that builds outboard engines and is developing an injection system for two-stroke engines that, according to rumors, is expected to be derived from the Orbital system. The company has not said much about this, but it has let it be known that two systems will be made available to its customers and both will be direct injection. On the medium-high power engines, an electromagnetic injector will be applied and thus the system could trace that of the OMC, while on the small power engines there is no talk of electronics but of a kind of “double-acting carburetor.” Knowing that Selva collaborates with Piaggio, one could speculate that this is a technology similar to the one that the Pontedera-based company has announced it will introduce on its new Vespa 50, the fuel supply of which is provided by a kind of “compressor” placed above the combustion chamber that is thus capable of directly injecting fuel by pulverizing it. Let’s not hide the fact that the system, as presented, has raised quite a lot of perplexity as it is quite complicated in its mechanics. Piaggio itself in announcing the release on the market of the Vespa 50 equipped with such an engine spoke only that it will be presented in the “next spring” without getting more specific about dates. We hope, however, that the new direct-injection Selva will arrive as soon as possible, if only to prove to the world that Italian technology has nothing to envy to American and Japanese ones.

by Furio Oldani

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