What is meant by 'Closed Loop Lambda Control'?
Why use large fuel injectors give poor exhaust emissions at idle speed?
If the Lambda sensor can accurately control the mixture, why is accurate fuel mapping required?
Thanks to Government legislation, over recent years, more attention is being paid to engine exhaust gas emissions, and within the performance tuning industry, this is viewed as either a good, or a bad thing!
To firstly attain acceptable emission levels, the engine must be capable of burning the fuel it uses efficiently. This is achieved initially by the basic mechanical design, (valve timing, port sizes, compression ratio's etc.) of the engine, and by the accurate control of fuel & ignition timing requirements. Finally, to maintain these emission levels, the condition of the engine and its ancillary components must be preserved.
Additional components (such as catalytic converters,) can be used after the engine, to 'clean up' exhaust gas emissions, but they can only work if the original engine is working efficiently.
With sufficient knowledge, and the use of the required sophisticated equipment, providing a high performance engine conversion that will meet all current exhaust emission test requirements is not difficult. However, without the knowledge to select the required mechanical components, and the ability to accurately tailor the engine management system to the mechanical engine specification, correct emissions cannot be achieved.
The ability to pass stringent exhaust emission tests isn't the only benefit to be gained by this, if efficient combustion of the fuel is achieved, then the maximum effect of this combustion will give you ultimate power for each unit of fuel used, assisting engine response, fuel economy, and of course, outright horsepower.
As explained above, correct exhaust emissions will only attained by an engine if the fuelling & ignition requirements are closely matched to the mechanical engine specification. It should also be noted that if the vehicle is fitted with a catalytic converter, this can only operate efficiently if the air/fuel mixture is at the correct ratio. Non catalysed cars rely upon the fuel mapping to be correct at all engine load & speed settings, ensuring that the air/fuel ratio is correct. On catalysed cars, an additional sensor is fitted to help maintain the air/fuel mixture at the predetermined, optimum level. This sensor is known as a Lambda sensor, and on a Cosworth, it will be found fitted into the turbine housing, at the back of the turbocharger. This device will measure the oxygen content of the exhaust gas, and send this information (as an electrical signal) to the engine management system. The management system will then make a calculation based on the Lambda signal, and determine whether the mixture is too rich, or too lean. Based upon the result of this calculation, the management system may then decide to increase, or decrease, the fuel fed into the engine at that point, to richen up, or lean off, the mixture. The chemically correct ideal air/fuel ratio is 14.7:1 by weight, (this is known as the Stoichiometric point), and is said that this equals Lambda 1. A richer mixture (less than 14.7:1) is below Lambda 1, and a lean mixture is above Lambda 1. In the UK, emission tests are carried out as part of the annual MOT tests. Increasingly popular now are spot checks carried out at the roadside. If your car fails a roadside test, you will receive an instant fine, penalty points on your driving license. In addition, you will receive instructions to obtain acceptable emissions test results from a registered MOT testing station, within a set period to prevent further prosecution. (As this was written this amounted to a £60.00 fine, 3 penalty points, and 14 days to present test results.)
For the Cosworth range, vehicle emissions test requirements can be divided into two categories, up to the end of July 1992, (D, through to J registration), and August 1992 (K registration) and onwards.
The earlier cars, (which weren't usually fitted with catalytic converters), have a far simpler emissions test. This is known as a non-CAT test, carried out at idle speed only, and just two gasses are measured. Carbon monoxide (CO) emissions should not make up more than 31/2% of the overall exhaust gas, and particles of hydrocarbon (HC) must not exceed 1200 parts per million. For the later model cars, (K registration onwards), the requirements are more stringent, legislation ensured that when these vehicles were first manufactured, they came complete with catalytic converters to 'clean up' the exhaust emissions. (Further explanation about the workings of Catalytic converters can be found later). This test is carried out at both normal idle speed, and a predetermined fast idle speed, and the engine oil temperature is measured to ensure correct engine temperature. In slow & fast idle tests, CO & HC readings are taken as before, but also the Lambda value of the exhaust gas is calculated & reported. To pass this test, the vehicle must provide exhaust emissions that fall within the original values provided by the manufacturer. For the Escort Cosworth the minimum figures are as follows: slow idle speed (825 - 950 RPM) CO: 0.5%, fast idle speed (2800 - 3100 RPM) CO:0.3% HC:200 parts per million, Lambda: 1.09 - 0.95 oil temperature 80°C.
Closed loop is the description of the constant control system employed by the engine management system. When the management system receives its signal from the Lambda sensor, it will then adjust the air/fuel ratio, (if required), and then compare the new Lambda reading to pre set levels. The management system will continue this process, and so it is said to be in a loop, looking at the mixture, adjusting the mixture, looking at the mixture, adjusting the mixture, and so on. (Closed loop control can also be used for turbocharger boost levels).
Why do large fuel injectors give poor exhaust emissions at idle speed?
The fuel injector is basically an electronically controlled tap & spray mechanism. Larger size injectors have larger sized nozzles. Injectors have only two positions, fully open, or fully closed, it is not possible to only half open an injector. To control the quantity of fuel provided, the injector is held open for a number of milliseconds by the engine management system. It is a basic fact that a known quantity of fuel is required for a certain level of horsepower, and the injector size (or flow rate) should be determined to provide this in the time allowed. For example, at 6000 RPM, there are only 20 milliseconds (0.020 seconds) in-between the ignition of each cylinder. If the fuelling requirements require a large injector to give high power at high engine speeds, this will require a very short opening duration at low engine speeds. In effect, a large, thin, heavy mass of fuel will be injected into the inlet air. As this will be unable to atomise fully within the air, not all of the fuel will be burned during combustion process, and poor emissions will result.
If the Lambda sensor can accurately control the mixture, why is accurate fuel mapping required?
In an ideal world, it would be nice to allow the management system to work out its own fuelling maps. Accurately calibrating the fuel map is an extremely time consuming operation. There are a couple of reasons why accurate fuel mapping is still required. Firstly, the Lambda sensors used on production vehicles are what is known as 'narrow band' Lambda sensors. This means that they only have a small operating window either side of the optimum setting. Once the fuelling is outside of this, the sensor cannot measure the mixture, and the management system is unable to make the necessary adjustments. Secondly, there are a number of occasions where the engine performance will be improved by not running at the Stoichiometric point. Under full load conditions, more power is produced if the mixture is slightly richer, and on the overrun, fuelling is often switched off completely to save fuel, and increase the engine braking effect.
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