Subaru Impreza RB320oc Owners Club RB320

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Unveiling the hidden beauty of Subaru's mechanical parts

Mechanical Parts

Engine control module



In this era of widespread computer use, everyone knows that cars have computers, and we consider this natural.

But there certainly aren't many people who understand precisely that the engine control module (ECM) consists of the computer and its related parts.

We rarely hear why today's cars even have an ECM, or that the use of ECMs in automobiles was the first step toward responding to the trend of the times. In fact, understanding the significance of the ECM is an important part of understanding the modern automobile, yet the regular driver has not had much of an opportunity to consider this point.

I thought this when I talked to Takashi Matsuura, who works in the research and development of electronic engine control systems at the Subaru Engineering Division.

"Subaru began controlling engines electrically some 25 years ago. Our objective was to meet new exhaust gas regulations at the time. Without ECMs we could not have controlled the output of exhaust gas to the current level." The best way to think of its importance for automobiles is that the ECM emerged as the automobile's savior.

I was given a look at the ECM for a Horizontally-Opposed 6 cylinder engine, to see for myself. It was a 15 cm square and 3 cm thick metal box, and it is placed under the floor where the front passenger's feet rest. Removing the cover reveals a unit consisting of a single printed circuit board with numerous electronic components on it.

"This is a microcomputer," says Matsuura as he points to a 2 cm square flat black component. "This computer processes all the information it needs to issue control instructions to the engine. The other components turn the computer's instructions into electronic signals."

So what work does this ECM perform? First, it controls the timing and amount of gasoline injection. It also governs ignition timing, and then controls the valves of the variable intake system and idle speed. Thus, the ECM optimizes engine running conditions. The ECM always comprehends engine conditions, and accurately controls the amount of gasoline needed. This is how it raises fuel efficiency and eliminates excess exhaust.

In addition, the engine ECM exchanges information with the transmission ECM to control engine revolutions precisely to eliminate the shock of the gears shifting. The engine ECM also exchanges information with the ECM that monitors the car's body, and so if, for example, the tires slip, the ECM cuts back on engine output. In addition to cooperatively exercising control in tandem with the ECMs for other parts of the car, the engine ECM diagnoses malfunctions.

The ECM's role in regulating the timing and amount of gasoline injection is easy enough to understand, but the fact that it exercises this control in millionths of a second indicates a broader unseen world.

"Idle speed control is the control of engine revolutions. For example, since the engine is cooled soon after it ignites, the ECM increases the number of revolutions, and then decreases them once the engine has warmed up," says Matsuura.

I thought this explanation made things clear, but greater detail reveals even more surprises.

"Out on the road, the driver will stop at intersections and so on, at which time the driver takes his or her foot off the accelerator. At this point, engine revolutions are irregular, affected by temperature and air pressure. The ECM senses temperature and air pressure, interprets the data, and stabilizes engine revolution. Or take the example of the pistons. As they compress and release exhaust, the movement speeds of the pistons display slight differences. The ECM senses such differences for each cylinder and controls them," says Matsuura. The ECM's job is both highly precise and broad in scope!

But this raises the question, how does the ECM relate to the driver? Does the ECM disregard the driver's will?

"Indicators respond to the driver pressing down on the accelerator, and the ECM instantly exercises control to create the optimum engine conditions according to the driver's will. Subaru vehicles are created to be fun to drive, and are programmed for that purpose so that they do not disregard the driver's will and ruin the driving pleasure. If the engine doesn't faithfully execute the driver's will, naturally driving would not be at all fun," says Matsuura.

The actual performance of the ECM components of all car manufacturers is similar. The difference lies in the programming of the ECM, and Subaru has confidence in its programming capabilities. ECM engineers conduct exhaustive tests over repeated test drives to program the ECM so that it is totally faithful to the driver's will and thus contributes to a fun driving experience. "Even in the case of a computer, such as the ECM, we carry out development by taking the car on an actual test drive. No matter what part of the vehicle we target, Subaru's development technique doesn't change."

"In the near future, a computer will collect data on a driver's habits and then optimize engine control according to them. That will further raise fuel efficiency and reduce exhaust," says Matsuura.

The ECM directly links to the cars of the future. In 30 years, it is possible that mainstream automobile power plants will change from engines to combinations of fuel cells and electric motors. Advances in ECM technology will be the key to this evolution.

"We electronic control engineers want drivers to better recognize the importance and potential of electronic controls. But a driving world in which drivers are conscious of the existence of the ECM would not be much fun or enjoyable. We want drivers to recognize our work, but we don't want the ECM to be at all noticeable. So I guess we'll just have to suffer," says Matsuura with a self-effacing smile.

I felt I could trust this smiling face. The face was full of absolute certainly that while electronic control will advance, the world of Subaru driving pleasure will never change.

Quick Links

Richard Burns

Subaru rally legend, Richard Burns Britain's only FIA World Rally Champion died in November 2005 of brain cancer at the age of 34.

Click here for more information on the 2001 World Rally Champion.

RB320 Features

  • 6,000 of extra equipment
  • 320PS/450Nm Prodrive tuned
  • build limited to 320 units
  • finished in Obsidian Black
  • bespoke 18" alloy wheels
  • unique numbered tax disc holder
  • Prodrive tuned suspension system with Bilstein dampers, Eibach springs & rear ARB
  • lowered ride height
  • polished s/steel mesh front grille
  • STI front lip spoiler
  • STI quickshift gearchange
  • STI side sill plates
  • RB branded gear knob
  • RB branded car mats
  • RB exterior badging
Click here for full list