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Fuel injection in economy cars While most electronic engine management systems in today’s cars are based around multipoint fuel injection, the Bosch Mono-Jetronic is based on just one injector and no airflow meter or MAP sensor. It is used in the Mazda 121 and some other small economy cars. By JULIAN EDGAR The majority of today's EFI systems use one injector for each of the engine’s cylinders. These so-called multi-point systems have the advantage of allowing the fuel to be added just before the inlet valves, giving benefits in mixture accuracy and overcoming manifold wall wetting. However, the cost of such a system is higher than that of a single-point system which normally uses only one or two injectors. 10  Silicon Chip In the cost-sensitive small, economy car sector, every extra dollar saved is crucial. If the injector count can be more than halved and at the same time the airflow meter or MAP sensor done away with, the cost of the system can be made very low. Unfortunately, the technical compromises implicit in a single point system require complex engineering solutions, if the car is to perform at a level near to that which would be achieved by a more expensive system. This article looks at how Bosch engineers developed a simple, cheap EFI system using just one injector and only four major input sensors. Their approach is also used when aftermarket programmable EFI systems are fitted to very “hot” piston engines and peripheral ported rotary engines. In these cases, a manifold pressure signal is not a reliable indicator of engine load and airflow meters are only rarely used. System layout On paper, the Bosch Mono-Jetronic appears similar to any of the more common EFI systems. Fuel is pressurised by an electric pump, fed through a fuel filter and then fixed at a level above the manifold air pressure by a fuel pressure regulator, before being fed to an electronically-controlled injector. Induction air passes through Fig.1: in the Mono-Jetronic system, many normally-discrete components are integrated into one unit: (1) fuel injector (2) intake air temperature sensor (3) throttle butterfly (4) fuel pressure regulator (5) fuel return (6) fuel inlet (7) throttle position sensor (hidden) (8) idle air bypass motor (Bosch) a filter, is monitored by an intake air temperature sensor and then it passes through the throttle body into the engine. However, as Fig.1 shows, the physical layout of the system is quite unusual. The fuel injector, air temperature sensor, fuel pressure regulator, throttle valve, idle speed control actuator and throttle position sensor are all integrated into one unit. Combining the various components into one package in this way obviously reduces manufacturing and installation costs. The assembly is positioned in a similar location to that used by a carburettor in an old car – on top of a multi-branch intake manifold. Collecting engine data The two major inputs determining the injector pulse width are engine speed and throttle position. Engine speed is easily derived by monitoring the ignition signal but accurate sensing of throttle position is more difficult. When load sensing is derived by monitoring the throttle angle, the relationship between the throttle valve opening and the flow area within the throttle body must be maintained to within very close tolerances on all Fig.2: the single injector is located directly above the throttle butterfly, with the fuel pressure regulator incorporated into the same housing. The rest of the fuel supply system is similar to any other EFI system: (1) fuel tank, (2) electric fuel pump, (3) fuel filter, (4) fuel pressure regulator, (5) fuel injector, (6) throttle butterfly. (Bosch) production units. This is because small throttle movements can make huge changes to the engine load. The first step in developing the system is to subject the engine to accurate dynamometer testing. This is so that the air charge for one intake cycle at various engine speeds and throttle openings can be measured. Fig.3 shows an example of these “air charge” amounts. Several interesting aspects can be noted about Fig.3. First, the amount of air breathed per intake stroke is at its maximum at peak torque, as is shown by the air charge line indicative of full throttle (the butterfly open by 90°). As can be seen, the greatest ingestion per intake stroke occurs on this engine at about 3000 rpm. However, of more importance when attempting to measure the correct amount of fuel to be added are the differences in air charge amount which occur at small throttle openings. At idle and low-load, a change of ±1.5° in throttle opening causes an air-charge difference of ±17%! On the other hand, the same amount of throttle movement at high loads can cause a change of only ±1%. From this, it follows that small throttle openings must be measured with extreme accuracy. In the Mono-Jetronic system this is carried out by an unusual throttle position sensor (TPS). All other EFI sysJuly 1996  11 Fig.3: an ‘air charge’ map is developed on an engine dynamometer to show the amount of air ingested during one cycle at different rpm and throttle openings. Note that at idle and low loads, a change of ±1.5° in the throttle opening causes an air charge difference of ±17%, while the same amount of throttle movement at high loads causes a change of only ±1%. This means that very accurate throttle position sensing is required. (Bosch) Fig.4: schematic diagram of the Mono-Jetronic ECU. (Bosch) 12  Silicon Chip A single point injection system can have major problems with manifold wall-wetting, even with a very finely atomised fuel spray. Mono-Jetronic uses sophisticated techniques to overcome these potential problems. tems also use a TPS but it is often just a two-position switch, with contacts for idle and full throttle. The Mono-Jetronic system uses two potentio-meters in its TPS. Each wiper arm carries four wipers, each of which contacts one of the potentiometer tracks. Track 1 covers the angular range from 0-24°, while Track 2 covers the range from 18-90°. The angle signals from each track are each converted by dedicated analog/ Fig.5: this Lambda Map shows the injection duration which gives a 14.7:1 air/fuel ratio at all loads and engine speeds. This is the actual base map, with the injector pulse widths then modified on the basis of the inputs of the other sensors. (Bosch) digital converter circuits. The ECU also evaluates the voltage ratios, using this data to compensate for wear and temperature fluctuations at the pot. Because the engine load cannot be assessed in this way as accurately as with MAP sensing or airflow metering, the system requires the feedback of an exhaust gas oxygen (EGO) sensor, if it is to comply with emissions legislation. The EGO sensor is the normal type, where the output is a small voltage which changes rapidly either side of the stoichiometric (14.7:1) air/ fuel ratio. Other sensor inputs include coolant, intake air temperature and control signals from the air conditioning and/or automatic transmission. The latter two inputs are used as part of the idle speed control. Processing of input data Fig.4 shows a schematic diagram of the system’s ECU. The inputs from the TPS, EGO, engine temperature and intake air temperature sensors are converted by the analog to digital converter and transmitted to the microprocessor by the data bus. The microprocessor is connected through the data and address bus with the EPROM and RAM. The read memory contains the program code and data for defining the operating parameters. In particular, the RAM stores the adaptation values developed during Fig.6: this graph shows the intake air temperature correction to the injector pulse width. Note that the system is calibrated to work over a 100°C range! (Bosch) self-learning, which occurs on the basis of the EGO sensor input. This memory module remains permanently connected to the vehicle’s battery to maintain the adaptation data whenever the ignition is switched off. A 6MHz quartz oscillator provides the stable basic clock rate needed for arithmetic operations. A number of different output stages are used to generate the control signals for the fuel injector, the idle speed control actuator, the carbon canister purge valve (which allows the burning of stored petrol tank vapour) and the fuel pump relay. The fault lamp warns the driver of sensor or actuator problems and also acts as a diagnostics interface. Mixture control The starting point for the calculation of the fuel injector pulse width is a stored 3-dimensional map derived from dyno test data. This “Lambda Map” (Fig.5) contains the optimum pulse widths to deliver a stoichiometric air/fuel ratio under all operating conditions. It consists of 225 control co-ordinates, made up of 15 reference co-ordinates for throttle position and 15 for engine rpm. Because of the extremely non-linear shape of the air-charge curves, the data points are situated very closely together at the low-load end of the map. The ECU interpolates between the discrete points within the map. If the ECU detects deviations from stoichiometric air/fuel ratios and as Fig.7: because only a single injector is used, manifold wetting can cause major problems during transients. Acceleration enrichment (1) and deceleration lean-off (2) is used, with both based on the speed of throttle movement. (Bosch) July 1996  13 Fig.8: the mixture signal from the exhaust gas oxygen sensor is used as a correction factor. Note that the greater the length of time for which the mixture is rich (or lean), the greater the amount of correction which is applied. (Bosch) a result is forced to correct the basic injection duration for an extended time, it generates mixture correction values and stores them as part of the adaptation process. In this way it can compensate for engine-to-engine variations and engine wear. However, because the Lambda map is designed only for the engine’s normal operating and temperature range, it becomes necessary at times to correct the base injector pulse widths. The first of these is when starting. Because the Mono-Jetronic system uses just one injector, manifold wall wetting through condensation is a much bigger problem than in multi-point systems. As in all EFI systems, injector pulse width is increased when the engine is cold but because Very “hot” engines using radical cam specifications sometimes make use of just engine speed and throttle position inputs to calculate the required fuel addition. Doing so overcomes the problem of the poor vacuum signal at low loads which can occur with high valve overlap. The system described here takes the same approach but for reasons of economy. 14  Silicon Chip condensation of the fuel also depends on the air velocity, the starting injector duration is reduced as engine speed increases. To counteract the possibility of flooding, the longer the engine cranks, the less fuel is injected; it is reduced by as much as 80% after six seconds of cranking. Once the engine has started, the injector opening duration is based on values stored within the Lambda map, suitably modified on both a time and temperature basis by the engine coolant temperature input. As the temperature of the intake air increases, its density is reduced, meaning that at a constant throttle position the cylinder charge reduces with increasing temperature. Fig.7 shows the relative enrichment at different intake air temperatures. Transition compensation While all EFI systems use the equivalent of a carburettor accelerator pump during rapid throttle movements, the single injector of the Mono-Jetronic system makes this a critical aspect. During sudden changes in throttle position, three factors need to be taken into consideration. First, fuel vapour in the central injector unit and intake manifold is transported very quickly, at the same speed as the intake air. Second, fuel droplets are generally transported at the same speed as the intake air but are occasionally flung against the intake manifold walls, where they form a film which then evaporates. Third, liquid fuel is transmitted as a film on the intake manifold walls, reaching the combustion chambers after a time lag. At idle and low loads, the air pressure within the manifold is low (there is a high vacuum) and the fuel is almost entirely vapour with no wall wetting. When the throttle valve is opened, the intake manifold pressure rises and so does the proportion of fuel on the manifold walls. This means that, when the throttle is opened, some form of compensation is necessary to prevent the mixture becoming lean due to the increase in the amount of fuel deposited on the walls. When the throttle is closed, the wall film reduces and without some form of leaning-compensation the mixture would become rich. Rather than basing the transitional compensation on throttle position alone, the system uses the speed with which the throttle is opened or closed as the determining factor. Fig.8 shows this compensation, with the maximum correction occurring when the throttle is opened at more than 260° per second. Also incorporated in these dynamic mixture corrections are inputs from the engine and intake air temperature sensors. If you are seeing a blank page here, it is more than likely that it contained advertising which is now out of date and the advertiser has requested that the page be removed to prevent misunderstandings. Please feel free to visit the advertiser’s website: www.telstra.com.au Mixture adaptation The mixture adaptation system uses the EGO sensor input. The system must compensate for air-density changes when driving at high altitudes, for vacuum leaks after the throttle butterfly and individual differences in injector response times. Fig.8 shows the variation in the Lambda correction factor with different EGO sensor output voltages. Updates occur at between 100 milliseconds and one second, depending on engine load and speed. Acknowledgment: thanks to Robert Bosch (Australia) Pty Ltd for providing much of the information used in this article. SC July 1996  15