Why people need a specialist ...

Engine Management Systems, Evolution

Pre 1980’s - Engine Management was performed mechanically by coil and distributor ignition switched by contact breaker points, fuelling was dealt with by carburettor. These systems were very straightforward and maintenance and repair was well in the scope of Garages and DIY Mechanics alike.

Early 80’s - Electronic ignition became more common with the humble contact breakers being replaced by an electronic switching. Fuelling was by and large still carburettor controlled but now there were more add on electrical devises, such as anti dieseling valves and idle control valves. Positive closed breather systems were used to improve emissions.

Fuel Injection was being more widely used but mainly on sports versions of models. Later Bosch versions had ECU control for the fuel injection whereby the injectors were electronically triggered by the ECU and the amount of fuel injected was based upon inputs from various engine sensors. The Peugeot 205 Gti being a common example of this.

Early 90’s - Emissions regulations were tightening and the first fully mapped systems were born. Ignition timing and fuel mixture were beginning to be out of the scope of manual adjustment, their mapped values stored on an ECU (Electronic Control Unit), more engine sensors were being employed to measure more information in order to tightly control emissions.

By the mid 90’s. fully Closed Loop systems were mandatory as Catalytic Converters were now in place. The term closed loop is defined by the fuelling being maintained to a target Lambda value by an Oxygen Sensor positioned in the exhaust system, somewhere between the exhaust valve and the catalytic converter. The sensor monitors the amount of oxygen in the exhaust gas and tells the ECU to alter the injected fuel either richer or leaner to maintain a target of Lambda 1 which represents the theoretically perfect Air / Fuel Ratio of 14.5 to 14.7 parts air to 1 part fuel.

This constant communication backwards and forwards between oxygen sensor and ECU forms a loop, hence the term closed loop. The purpose of the Catalytic Converter is to break down the harmful CO, HC and NOX emissions and convert them into less harmful O2 and CO2, this is done by using certain precious metals built into a dense brick with a honeycomb cross section which react positively when our chemically correct air/fuel ratio is flowed into it. The most effective chemical conversion occurs when the air/fuel ratio is at 14.7:1 which is why it is important to keep the fuelling under closed loop control to optimise best engine emissions.

Another way of optimising engine emissions is to keep closer control over the ignition spark. As closed loop fuelling came into commonplace so did mapped spark. Earlier 90’s systems had mapped spark that triggered a conventional ignition coil, the coil discharging a High Tension spark via a mechanical distributor to the appropriate spark plug. Later came distributor less systems whereby mapped spark is sent straight from the ECU to individual cylinder coils that fire the plugs directly.

Other emissions devices found on cars built from the mid nineties to the present include,

Exhaust Gas Re-circulation, that is where an amount of exhaust gas is bled off from the exhaust manifold and fed back into the inlet system to mix with a fresh charge of air/fuel. The advantage of doing this is to lower the combustion temperature and pressure of the inlet charge which reduces NOX emissions, on some applications this enables the manufacturer to run with more advanced mapped ignition with EGR (Exhaust Gas Re-circulation) which improves fuel economy.

Evaporative Control Systems, these are used to reduce the amount of HC emissions, a carbon filled canister is used at the end of the fuel vent line from the fuel tank and fills with fuel vapour as the tank de-gases, this occurs more frequently in hot operating conditions. When the engine is running, a vacuum is created in the inlet manifold which causes air to be drawn through the carbon canister drawing fuel with it, this mixture then is metered by a purge valve before re entering the engine for combustion. PHOTO 3 ROVER

Secondary Air Injection. These systems allow faster Catalytic Converter light off by pumping air via an electric pump into the exhaust system before the catalyst brick face during the engines warm up period, this reduces the HC and CO emissions.

Now and the future ...

We are now starting to see further advancements in electronic control as the next gateway for even more stringent emissions regulations is in 2007. Vehicles with both up and downstream lambda full feedback control which controls the fuel / air ratio even more tightly to a target lambda are now in place as well as engines with “torque Based Strategies” which consist of electronic throttle controls (no accelerator cable). These systems maintain a target engine torque based upon Driver demand, in other words wherever the driver puts the pedal, the engine ECU will calculate, taking in to account all other input data, what the engine torque should be. One main advantage of this system is that the target torque data can then be used by other vehicle systems like the transmission, traction control / ABS, the data is transmitted to the ECU’s of these other systems as a Network message on a CAN (Control Area Network) message. The CAN network basically shares common data that each system relies upon. Each system ECU is connected via a pair of wires that are capable of transmitting data continuously at high speed. The latest Vauxhall Vectra and Astra models utilise CAN messaging for all their systems, over the next few years all other Manufacturers will follow suit.

Diesel Technology

Diesel Engines fitted in modern cars have not escaped the rapid development in electronic control. In fact it could be argued that they have seen the biggest and most advanced changes. Through the eighties and up to the late nineties, fuel management on diesel passenger cars was limited to a mechanically driven (normally via cambelt) rotary low pressure pump. The only electronics involved were solenoid valves for fuel cut off, idle quality and in some cases engine immobiliser.

In the late nineties, diesel engines were fitted with electronic management systems with similar sensors as their petrol powered cousins all ECU controlled. Mechanical injectors were replaced with electronic ones triggered by the ECU. However for this change to take place the overall design of the fuel injection system altered.

The mechanically driven rotary pump that was used to deliver fuel to each cylinder in turn was now being used to deliver fuel to a high pressure fuel accumulator. This pressurised plenum of diesel fuel has a high pressure fuel supply line to each cylinder, as the injectors are electronically switched, the high pressure fuel enters the engine. To compensate for the drop in accumulator pressure a regulator allows more fuel to enter via the rotary pump. These systems are known as “Common Rail Diesels” and are fitted to many modern passenger cars including the Peugeot Citroen HDI range.

Just as with petrol engines, the diesel engine emission level requirement has become and is becoming more stringent, so further technology is being used to lower HC, NOX and Soot Particulate emissions, EGR is used along with Particulate traps fitted in the exhaust system plus more accurate control of fuel injection timing.

ECU Diagnostics

As electronic control of modern Engine management Systems has escalated so to has the need to monitor the systems so they run efficiently. Modern ECU’s have the capability to constantly monitor their own operation and the various components that make up the whole system. If a component of the system fails to meet certain conditions under the systems self test process, then a fault code is stored in the ECU and in some cases the ECU will run at pre-programmed default settings in order to keep the engine safe.

Diagnostic Tools

There are many diagnostic tools available on the market all varying in quality, price and capability but at the end of the day they are all just tools and are as only as good as the person using them.

There are two types of diagnostic tool available that is reflected in the way they are used to test a system. These are serial tools and parallel test tools. Serial tools include tools known as “Scan tools”, “Code Readers” etc which plug directly into the vehicles diagnostic socket and read from the vehicles ECU. Parallel test tools include oscilloscopes and multimeters and are used to take data from various points of the system in a “piggyback” fashion. Both sets of tools have their specific uses, their positives and negatives.

The Need for a Specialist Repairer

From the very brief overview above it can be seen that anyone who owns a car built from the early 90’s to the present who encounters a problem with their car are going to need to seek the help of a Specialist Repairer. That Repairer must be able to meet the following criteria.

They must have a working knowledge of modern vehicle systems including what the parts are that make up a system, how they interact with one another and what the possible impact on vehicle performance a failure of a particular part can have.

They must have suitable serial and parallel test equipment to test the vehicle and know how to use it.

They must have a comprehensive reference library of data to refer to in order to compare results against.

If your chosen repairer fails to meet any of the above then the quality and durability of the repair is likely to suffer.