Direct fuel injection - where the spray of fuel occurs right into the cylinders rather than just the inlet ports - is now available on road cars in this country. The Alfa 156 JTS uses it, while in Japan many Mitsubishi road cars use the process. Audi calls their system FSI and also has it available on road cars in some areas of the world. BMW is using a direct injection V12 in their 760iL range-topper - in fact, in the not too distant future, direct injection is likely to be found on examples of every manufacturer's engines.
And it's not just in road applications that the technology is being successful. Audi used direct injection for the first time on a mainstream racecar with their 2001 Le Mans winning cars. The 3.6-litre V8s developed over 600hp, despite having to breathe through two 32.4mm restrictors and having the boost of the twin turbos limited to just under 10 psi. Critical in the adoption of the new direct injection technology was a resulting gain in fuel economy under high loads - the direct injection race car reduced fuel consumption by 8 per cent and additionally, also yielded up to 9 per cent more torque throughout the engine rev range.
This article - by the Manager of Development of the Engine Management Gasoline Division, Robert Bosch Gmbh - takes an overview look at the Bosch DI-Motronic system, the most common of the direct injection systems being fitted. Next week we will look at some engines using the system.
The Main Components of DI-Motronic
Bosch's high-pressure injection system for gasoline engines is based on a pressure reservoir and a fuel rail, which a high-pressure pump charges to a regulated pressure of up to 120 bar (over 1700 psi!) Because of this very high pressure, the fuel can be injected directly into the combustion chamber via electro-magnetic injectors. The airflow into the engine is adjusted through an electronically controlled throttle and is measured by an air mass flow meter. For mixture control, a wide-band oxygen sensor - which can measure a range between Lambda of 0.8 and infinity - is used in the exhaust.
DI-Motronic uses three operating modes:
1. Stratified charge operation, with Lambda values greater than 1 (ie air/fuel ratio greater than 14.7:1)
2. Homogenous operation, of Lambda of 1 (ie air/fuel ratio of 14.7:1)
3. Rich homogenous operation, with Lambda of 0.8 (ie air fuel ratio of 11.8:1).
In a direct injection system the available time for injection is significantly shorter during part-load, stratified charge operation - for example, injection times at idle are less than 0.5 milliseconds. This is only one-fifth of the available time experienced with conventional manifold injection. However, even in this very short time the fuel must still be finely atomized in order that it can create an optimal mixture in the brief moment between injection and ignition. Direct injection fuel droplets are on average smaller than 20 µm - only one-fifth of the droplet size used with traditional manifold injection and one-third of the diameter of a human hair.
Still more important than fine atomization is even fuel distribution in the injection beam, allowing the achievement of fast and uniform combustion. These injector demands can be fulfilled only with the latest, extremely precise manufacturing technology.
New injection nozzles from Bosch make it possible for the direct injection spark ignition engine to make the transition from wall- or air-guided to a spray-guided combustion process, like the diesel engine. The engines thereby achieve better efficiency.
Conventional spark ignition engines have a homogenous air/fuel mixture at a 14.7 to 1 ratio, corresponding to a value of Lambda = 1. Direct injection engines, however, operate according to the stratified charge concept in the part-load range and function with high excess air (ie very lean air/fuel ratios). In return, very low fuel consumption is achieved.
The fuel injection is directly into the combustion chamber, occurring just before the ignition point. The result is a combustible air/fuel mixture cloud on the spark plug, cushioned in a thermally insulated layer composed of air and residual gas. This raises the thermodynamic efficiency level because heat loss is avoided through the walls of the combustion chamber. The engine operates with an almost completely opened throttle valve, which avoids additional losses.
With stratified charge operation, the Lambda value in the combustion chamber is between about 1.5 and 3 (an air/fuel ratio of 22:1 - 44:1). When compared to conventional gasoline injection, gasoline direct injection achieves the greatest fuel savings in the part-load range, with up to a 40 percent reduction in fuel consumption at idle.
With increasing engine load (and therefore increasing injection quantities) the stratified charge cloud becomes richer and emissions characteristics worsen. Like diesel engine combustion, soot may form. To prevent this, at a defined engine load the DI-Motronic engine control converts to a homogenous cylinder charge. The system injects very early during the intake process in order to achieve a good mixture of fuel and air at a ratio of Lambda = 1 (14.7:1 AFR).
As is the case for conventional manifold injection systems, the amount of drawn-in air is adjusted through the throttle valve, according to the desired torque requested by the driver. The DI-Motronic system calculates the amount of fuel to be injected from the drawn-in air mass and performs an additional correction via Lambda control (using the wideband oxy sensor). In this mode of operation, a torque increase of up to five percent is possible. Both the thermodynamic cooling effect of the fuel vaporizing directly in the combustion chamber, and the higher compression of the engine, play roles in this.
To be able to fulfil these different operating modes, two major requirements are made of the engine management system:
- The injection point must be adjustable between "late" (during the compression phase) and "early" (during the intake phase).
- The adjustment for the drawn-in air mass must be separated from the accelerator pedal position so that unthrottled engine operation in the lower load range, and throttle control in the upper load range, can both occur.
With optimal use of the advantages, the average fuel savings in the European Driving Cycle is up to 15 percent.
In stratified charge operation (ie when using very lean mixtures), the oxides of nitrogen (NOx) amounts in the exhaust cannot be reduced sufficiently by the action of a conventional, three-way catalytic converter. While the NOx can be reduced by approximately 70 percent through recirculation before the catalytic converter, this is not enough to meet coming emissions legislation. Therefore, emissions containing NOx must undergo special subsequent treatment. Today, engine designers are using an additional NOx accumulator catalytic converter in the exhaust system to deposit the oxides of nitrogen in the form of nitrates (HNO3) on its surface, with the oxygen still contained in the lean exhaust.
But the capacity of the NOx accumulator catalytic converter is limited. As soon as it is exhausted, the catalytic converter must be regenerated. In order to remove the deposited nitrates, the DI-Motronic system briefly changes over to its third operating mode: rich homogenous operation with Lambda values of about 0.8 (~12:1 AFR). The nitrate, together with the carbon monoxide (CO), is reduced in the exhaust to non-harmful nitrogen and oxygen.
The engine management system therefore has the difficult task of changing between the two different operating modes in a fraction of a second, in a way not noticeable to the driver.
The Starting Points for Better Efficiency
Experts calculate the overall efficiency of previous spark ignition engines in the European test cycle at 13 percent. In other words, only about one-eighth of the energy of the gasoline can be converted into useable motion energy.
This 87 percent loss has very different causes:
- Even if the engine process was ideal, energy losses of 45 percent would result in the stratified charge operation.
- For the engine in Lambda = 1 operation, another 7 percent loss is added.
- Losses from "non-ideal combustion" and heat losses on the combustion chamber walls cost an additional 15 percent.
- Ten percent is from throttle loss.
- The friction in the engine and the necessary auxiliary aggregates makes up 10 percent.
In order that these losses are minimised, the Bosch designers have addressed three issues:
- About five percent can be gained by using a stratified charge operation at Lambda values greater than 1 (AFR leaner than 14.7:1) for periods.
- Another five percent of losses can be avoided from "non-ideal combustion and wall heat losses".
- Through frequent unthrottled operation, an additional five percent of losses can be reduced.
The improvement in efficiency able to be gained from gasoline direct injection - resulting in fuel savings or higher power - has been known about for a long time. But until very recently, this process could not be used for spark ignition engines because of limited engine power with stratified charge operation and the nitrogen oxide emissions occurring in the lean exhaust.
The technology to overcome those problems has now arrived.
Next week: a look at some current direct injection engines
The First High Performance Direct Injection Car
The legendary Mercedes 300 SL, presented in 1954, was the world's first production passenger car with a four-stroke engine and Bosch gasoline direct injection.
The six-cylinder 215hp engine fired the gullwing coupe up to speeds of 260 km/h. The combination of tubular grid-type frame and gullwing doors was without precedent; the low, stretched hood with its characteristic power domes, the distinctive splash guards above the wheelarches, the perfectly balanced, elongated and elegant bodywork lines - all these were the hallmarks of a dream car.
After the first tests, the press was beside itself. The German trade journal auto, motor und sport wrote: "Among the sports cars of our time, the Mercedes-Benz 300 SL is both the most refined and the most fascinating - a dream of a car."
The British journal AutoSport enthused: "The Mercedes-Benz 300 SL is a car with a wonderful external appearance, coupled with virtually unbelievable performance. Its design and production quality border on perfection and the entire concept represents the uncompromising realization of all the new ideas the car incorporates."
Road & Track in the United States was equally full of praise: "When a comfortable interior combines with remarkably good handling, with almost terrifying road-holding, a light and at the same time precise steering and a performance that matches or even exceeds that of the best cars to date, then there's only one more thing to say: The sports car of the future has become reality."
The Mercedes-Benz 300 S, launched in 1951 and the last representative of a legendary coupé and convertible tradition in frame construction, also featured a petrol injection system in the last version to be built from 1955, but with a tamer nature. This six-cylinder engine delivered a moderate 175hp and permitted a top speed of 180 km/h.
From 1957 onwards, a new fuel system technology was adopted. This was the indirect petrol injection system - or port fuel injection - where the atomised fuel was no longer injected directly into the combustion chamber, but instead into the manifold. The new technique was first incorporated in the Mercedes-Benz 300 d, the last of the four versions of the 300 series built until 1962.
Until the recent re-introduction of direct injection, all fuel injected cars of the last 40 years have used port injection.