Suzuki's Twin Swirl Combustion Chamber design, (TSCC), improves cylinder charging, 
fuel combustion efficiency and throttle response on Suzuki stroke engines equipped 
with 4-valves per cylinder. The unique shape of the cylinder head, combined with two 
intake valves, causes the incoming fuel/air mixture to form separate high speed swirls.
 The swirling fuel/air mixture is accelerated by the wide squish area of the combustion 
chamber resulting in rapid combustion of the fuel mixture. The spark plug is centrally 
located between all four valves which also assists in rapid burning of the fuel mixture.

The Twin Dome Combustion Chamber design, (TDCC), has similar performance features and advantages as TSCC, but is designed for 2-valve per cylinder engines. The combustion chamber is shaped with two domes that set up the incoming fuel/air mixture in a high speed swirling motion. The combustion chamber has a wide squish area that assists in rapid and efficient combustion of fuel/air mixture, resulting in quick throttle response and good engine power.


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Combustion Chamber Developments
Help Reduce Emissions By Jeffrey Kidder and Randy Potter
The natural gas Industry is being presented with a growing number of options for reducing emissions from its medium speed reciprocating engines. For those operating in the strict emissions regulation areas, the option is still increasing the combustion airflow (through turbocharging) and using pre-combustion chambers to ignite the resulting lean air/fuel mixture. Recent developments have led to what some consider a more economical and user friendly pre-combustion chamber that screws into the existing spark plug hole. Original Engine Manufacturers (OEMs) have offered bolt-on pre-combustion chambers for years; some have also developed screw-in chambers. However, several after-market companies are developing patented screw-in pre-combustion chambers, as well. Some are being used to meet stringent emissions regulations. In addition to extending lean limit operation with pre-combustion chambers, there have been improvements to in-cylinder mixing, resulting in more efficient combustion. Testing has indicated that increasing the pressure at which the fuel is injected, and directing the dispersion of the fuel during injection can significantly reduce emissions and fuel consumption. 
CAAA Impact
The Clean Air Act Amendments of 1990 call for pollutants from these sources (NO, CO, THC's, VOC's, etc.) to be subject to more stringent regulatory requirements. In many cases, emissions must be reduced to as little as 10 to 20 percent of uncontrolled levels. Emissions reduction is now competing with efficiency and reliability as priorities during the research and development phases of new horsepower. There are two fundamental avenues for reducing emissions: after treatment of the exhaust; and modification of the combustion process. These avenues are best illustrated by the different solutions typical for the smaller high speed engines vs. the larger medium speed engines. Non-Selective Catalytic Converters (NSCR) and Air-Fuel Ratio Controllers (AFRC) are commonly installed on high speed, rich burn units to achieve very low emission levels. The medium speed, large bore engines are more commonly retrofitted with lean burn conversions, which reduce emissions by adding excess air to the combustion process and improving the in-cylinder air/fuel mixing and ignition. The emissions from the larger, medium speed engines can be reduced also by using Selective Catalytic Converters (SCR), however, this technology introduces its own environmental and operational hurdles due to the handling and injection of ammonia (NH3). 
Emissions
Catalytic converters are the primary choice for after-treatment of the exhaust today. Non-Selective Catalytic Converters (sometimes called 3-way catalysts) effectively reduce emissions from 0.5 percent or less oxygen present in the exhaust stream. Oxidizing catalyst technology (for reducing carbon monoxide and hydrocarbons) is effective where the oxygen content is greater than 1 percent , however, NOx, cannot be reduced by catalysis in oxygen rich environments (.>1 percent). The Selective Catalytic Reduction (SCR) method of NOx control, where NH, is injected into the exhaust stream to react across a base metal catalyst, has proven successful in the reduction of NOx, however, the handling, storage and injection of ammonia presents operational, safety, and environmental difficulties. These difficulties have caused operators to use SCR technology as a last resort Excess oxygen in the exhaust stream of two-cycle, large-bore, reciprocating is a consequence of using air flow to scavenge combustion products and introduce a fresh change of air for the next combustion cycle. This air flow can provide additional benefits such as increased horsepower, increased cooling and lower emissions. Turbocharging, which results in compression and increase volume flow of the combustion/scavenging air flow, is the primary means used by today's lean burn packages. New or upgraded turbochargers supplying a higher pressure air flow to the power cylinders increasing the total mass of air trapped in the combustion process. Increased air flow through the engine increases the scavenging efficiency, and ensures the replacement of combustion products with a fresh charge of air. If the amount of fuel is not increased to increase the power output, the net affect is a higher air/fuel mixture. The increased mass in the combustion process absorbs more heat thus reducing the peak combustion temperatures. Because NOx formation is exponentially dependent on temperature, reduced peak temperatures can provide dramatic reductions of NOx emissions. The penalty of high air/fuel mixtures has typically been a higher heat rate(btu\bhp\hr), and often reduced combustion stability. These effects have been mitigated by matching the pre-chamber volume and nozzle configuration to the volume and mixing characteristics of the open combustion chamber. Additionally, investigations of improved mixing via higher fuel pressures and directionally controlled injection have led to furl injection valve designs which make it possible to maintain the specific heat rate while reducing emissions.  Air/Fuel Ratio
Air/Fuel Ratio is defined as the mass balance of air and fuel present in the time of combustion. A stoichiometric mixture is chemically optimal point where the ratio of fuel and oxygen molecules is exactly correct-thus, under ideal conditions, the oxygen and fuel molecules are completely consumed during combustion. The only products from ideal stoichiometric combustion are nitrogen (due to presence in air), carbon dioxide and water vapor. This stoichiometric point for any fuel is referred to as the relative air/fuel ratio 1 (l=1). The relative air/fuel ratio (l) is a measure of the actual air fuel ratio as compared to the stoichiometric ratio. Any number less than 1 is rich of stoichiometric (excess fuel), and greater than 1 is lean (excess oxygen). The relative air/fuel ratio l=1 for natural gas is approximately 17:1. Due to incomplete mixing, reverse reactions and the presence of nitrogen, byproducts are formed in addition to CO2, nitrogen and water are formed. Of concern are the oxides of nitrogen (NO and NO2-collectively referred to as NOx) and carbon monoxide (CO). Figure 1 shows the general relationship between relative air/fuel ratio and these byproducts. In two-stroke engine applications, the air/fuel ratio discussion is complicated by the use of air for scavenging. The explanation in Figure 1 is based on a "trapped" charge of air and fuel in the cylinder. To date, there is no practical means of measuring the trapped air/fuel ratio in two-stroke engines. Therefore, it has become standard to express the air/fuel ratio as a total engine number, including the air used for scavenging. Typical air/fuel ratios for the large bore, medium speed two-cycle engines in the field today range from 35:1 to 55:1. This number varies depending on engine make and model and the emissions levels the operator is trying to attain. 
Ignition and Mixing
The implementation of lean burn operation quickly presents a new set of hurdles, the tallest of which is the ability to ignite the air-fuel charge. As the air/fuel ratio increases, more energy is required to initiate combustion. Additionally, because of non homogeneous mixing it becomes less likely that an ignitable mixture of air and fuel will be present in the immediate vicinity of the spark plug. Multi-spark ignition systems have been developed by several major ignition system companies. These systems have proven to be an effective and economical means to achieve ignition of moderately lean mixtures. However, when more extreme air/fuel ratios are required, a higher energy ignition source is needed. Pre-combustion chamber development has focused on extending the lean limit of engines by introducing significantly greater levels of energy than is possible spark and plasma ignition sources. Pre-combustion chambers are typically configured as a small volume (1-3% of the clearance volume) external to the main cylinder volume. A controlled and easily ignitable air/fuel mixture is fired in the pre-chamber. The combustion in the pre-chamber causes a pressure rise and subsequently forces a jet of burning gases through the PCC nozzle into the main combustion chamber. Unlike the flame kernel from a spark ignition, the jet from a PCC introduces orders of magnitude more energy for primary ignition - thus providing the ability to ignite a much higher air/fuel ratio. Additionally, the surface area and penetration associated with the flame from the pre-chamber provides more rapid and complete combustion of the charge in the main chamber, thus allowing more efficient use of the fuel. The quality of the in-cylinder mixing of the air and fuel is as important a factor as the ignition source and the air/fuel ratio. In most two-cycle engines the air-fuel charge is not homogeneously mixed at the time of combustion. This is due to a number of factors: low velocity (and therefore, momentum) of the infected fuel, poor dispersion of the fuel into the air, cylinder head and piston shapes, port locations, etc. Pockets of poorly mixed air and fuel residing in the main chamber make it difficult to obtain consistent and efficient combustion. Steps toward improving the air and fuel mixing, have led to the development of fuel injection valves which direct and improve the dispersion of the fuel into the air. Fuel injection valve nozzle shapes have been modified to inject fuel at higher velocities (and therefore, momentum) for mixing, and in a pattern that more evenly disperses the fuel into the main combustion chamber.  
 
Recent Applications

In order to meet new regulations being implemented this year, Diesel Supply Company, USA, and a major pipeline company recently conducted a joint development project on a Clark TLA-6. In an effort to cost effectively meet the stringent emissions requirements of the local Air Quality Management District, this program included pre-combustion chambers and modified fuel valves. In order to evaluate emissions and general performance, testing was conducted through a matrix of varying Air Manifold Pressure (AMP), pilot gas pressure, ignition timing and engine load. Figure 2 illustrates the NOx emissions and heat rate vs. Air Manifold Pressure (AMP). The emissions measurements were conducted by an independent Reference Method van, the horsepower was indicated on a PFM 2000, the mass emissions were determined using EPA 40 CFR 60 method 19, and the fuel flow was measured with a roots type flow meter which had been calibrated specifically for the test. The engine was well tuned to begin with, and had previously been retrofitted with a high efficiency turbocharger: the NOx levels ranged from 4gms/bhp-hr at 14 in. Hg AMP, to 3.25 gms/bhp-hr at 19 in. Hg AMP and the heat rate ranged between 6500-6900 btu/bhp-hr in the baseline configuration. The baseline test condition for comparison testing was defined as rated horsepower (2000 hp), rated speed (300 rpm), and 19 in Hg AMP. Upon installation of the pre-combustion chambers and modified fuel valves there was an immediate reduction in NOx emissions and heat rate. At the baseline conditions, NOx emissions dropped approximately 1.4 gms/bhp-hr, and the heat rate improved to approximately 6750 btu/bhp-hr. The CO levels dropped from 0.8 to 0.5 gms/bhp-hr, however, the Total hydrocarbons (THC's) increased from 4.6 to 5.2 gms/bhp-hr. As expected, improved mixing, increased AMP, and the ability to burn the higher air/fuel mixture led to reductions in the emissions levels (total air/fuel ranged from 45 to 54 over the range of AMP's tested). At the maximum AMP which still exhibited acceptable combustion stability (26 in. Hg), the NOx, CO, THC, and VOC emissions were at approximately 0.5, 1.0, 9.0, and 0.2 
gms/bhp-hr respectively.   


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Lean burn combustion engines
The ratio of air to fuel in a spark ignition engine is on average 14.7:1. Beyond this ratio, the engine begins to misfire resulting in an unsteady combustion cycle. In the past the air - fuel ratio was kept above this figure, to prevent any combustion problems. Since the end of WW2 however, governments have been pushing the automotive industry to reduce car exhaust emissions. This, along with the realisation of diminishing oil reserves, have forced automotive designers to look at higher air - fuel ratio engines, in order to increase fuel efficiency. These higher than average air - fuel ratio engines are called lean burn engines and have been developing since the 1960s. 

The way that these engines work smoothly, despite burning air - fuel mixtures of about 18:1 is down to many new design features. The amount of fuel and air entering the combustion chamber must be closely monitored, as small variations when the mixture is so lean, can greatly effect the performance of the engine. The introduction of fuel injection combats this problem, as the spraying of fuel directly before the inlet valves provides particularly even mixture distribution. 

 

The mixture must also be injected into the combustion chamber in an intensive swirl. This concentrates the fuel around the spark plug which keeps the ignition duration from cycle to cycle constant, preventing misfiring. In order to concentrate the fuel around the spark plug, the combustion chamber is designed so the air motion swirls up towards the spark plug. Equally, the spark plug is placed in the centre of the combustion chamber. This places the spark plug closer to the fuel and prevents the fuel furthest away from the spark plug self-igniting. This means that the compression ratio can be increased and greater fuel efficiency can be achieved. 

The above improvements enable the air - fuel ratio to be increased and so decrease the specific fuel consumption of the engine by 5 - 35%. Lean burn engines also have the advantage of decreasing polluting emissions. The amount of carbon monoxide emitted is less, as there is plenty of oxygen available to produce carbon dioxide. Unburned hydrocarbon emission is reduced by up to 78% in lean burn engines. This is because unburned hydrocarbons are released when combustion is not complete. Fuel injection prevents fuel droplets settling in the intake passage, by spraying them directly before the inlet valves. It also produces a homogeneous air - fuel mixture. 

  

Conventional 2 stroke engine design vs. Direct injection. 

Both of which provide almost complete combustion for every cycle. Hydrocarbon emission can also be reduced if the combustion chamber is smooth and has a small surface area. This is because unburned hydrocarbons originate from crevices in and layers next to the combustion chamber walls. The combustion chamber in a lean burn engine is a small bowl shape, in order to promote the swirl airflow and to reduce the length of the ignition spark (so the compression ratio can be increased). The bowl shape also allows fuel ignited before reaching top dead centre to be ignited more evenly. 

 

The shape of a lean burn combustion chamber. 

The environmental drawback to higher air - fuel ratios is that lean burn engines produce nitrogen oxides. This is because nitrogen oxides are formed at high temperatures. The short ignition spark, the compact fuel in the combustion chamber and the high compression ratios all lead to high temperatures in the combustion chamber and so nitrogen oxide emission. New government regulations have set a 40% nitrogen oxide reduction target for the automotive industry, so improvements must be made in the design of the engines. Nitrogen oxide emissions can be improved by 95% if exhaust recirculation is used. This process, however, involves placing exhaust gases back into the air - fuel mixture. This reduces the temperature of combustion, which produces less nitrogen oxide emissions. The lowering of combustion temperature results in the emission of unburned hydrocarbons increasing and fuel economy decreasing. This method goes against the principles of lean burn engines, so an alternative needs to be found. Catalysts can also be used to reduce nitrogen oxide emissions by converting them to less harmful chemicals. 

 

The new Mitsubishi "ultra lean burn" engine uses a catalytic converter to reduce NOx emissions. 

Unfortunately, the three way catalytic converter needed to remove nitrogen oxides from the exhaust gases, will only work at a rich air - fuel ratio. 

As air pollution has become more important to automotive companies and governments, the lean burn combustion engine has developed and improved. Mitsubishi have created a lean burn engine that will operate a 40:1 air - fuel ratio mixture. 

 The new Mitsubishi engine 

The reason that they have been able to put this engine on the market is because the "ultra lean burn mode" is not used for starting the car or accelerating the car. For these tasks the engine uses a "superior output mode" which uses a lower air - fuel ratio to provide extra power. The reason that the lean burn engine is not used during start up, is because even in a lean burn engine, more unburned hydrocarbons and carbon monoxide emissions are created at this stage. This means that there is no advantage in using the lean burn, lower performance engine at the start up stage. The "ultra - lean burn" engine does, however contain new technologies. Instead of making the air - fuel mixture spiral around the combustion chamber. The Mitsubishi injector forces the mixture to tumble clockwise into the chamber, using upright and straight intake ports. This system breaks up as the cylinder compresses, forming eddies that push the fuel even closer to the spark plug. 

  

Diagrams of Swirl and Tumble flows 

A new injector system has also been introduced which provides a more reliable source of ignition for lean burn engines. The stratified charge system atomises the fuel across the spark plug. The particles of fuel produced are much smaller than conventional processes, less than eight microns as opposed to 27 microns in a high pressure direct injection system. 

 

Graph showing the size of fuel particles in stratified charge systems and conventional injection systems. 

When ignited, a fuel cloud is produced creating very high temperatures and a higher combustion percentage than in traditional lean burn engines. In this system, a low air - fuel ratio engine does not need to be used, when the car starts or accelerates. This is because the direct injection system is advanced enough to add enough fresh air to the fuel mixture, to stop fuel condensing in the air combustion chamber, preventing the engine from misfiring. 

There is some debate on the merits over two and fours stroke lean burn engines. Two stroke engines leave high temperature residual gases in the combustion chamber. This decreases unburned hydrocarbon emission. The combustion chambers in two stroke engines can also easily be converted to the lean burn "bowl" design, that is crucial to lean burn engine design. Four stroke engines however, permit central spark plugs and more compact combustion chambers, which also lead to reduced hydrocarbon emissions. From this argument I feel that two stroke engines are better suited to lean burn, as they are lighter than four stroke engines. A small, light weight engine has the advantage of increased fuel efficiency. 

 

A great advantage of two stroke engines is their reduced size and wieght compared to four stroke units. 

As the lean burn engine is primarily designed to reduce fuel consumption, I feel this is a very important feature. OCP (Orbital Engine Corporation limited) have, in fact designed a car using two stroke, lean burn combustion engine. It reaches the 40% reduction on nitrogen oxide emission and the 15% unburned hydrocarbon emissions, requested by governments for the new millennium. 

 

Graph showing the emissions of the OCP unit after 80,000 km against the proposed European requirement. 

One of the key reasons for this design being able to achieve such low emissions, is that it is so light. 

The government requirements mentioned above have created a large amount of research into lean burn combustion. Increased oxygen content in the air entering the combustion chamber would allow engines to run on leaner fuel. Membranes are therefore being developed by NCERQA - National Center for Environmental Research and Quality Assurance" that could increase the oxygen content in air as it entered the intake valve to the engine. The use of chaos theory in engines is also being researched by Dr Drallmeier and Robert Wagner". This is because the cyclic combustion variations are completely random, so, to develop a control system for lean burn engines, a way of predicting their behaviour must be found. A catalytic converter is being researched that will tackle nitrogen oxide emissions at lean air - fuel ratios. A new catalyst has been found - hydrous metal oxides - by Sandia technology - which is not depressed at high oxygen levels. This makes it ideal for lean burn engine emission control. Hydrous metal oxides are good catalysts, as they have a high cation exchange capacity, a high surface area and can be used as a coating or a bulk material. At present different hydrous metal oxides are being tested for the best nitrogen oxide reduction. 

As oil reserves continue to reduce and air pollution becomes more of a problem, fuel consumption and the exhaust emissions of cars must be reduced. The use of higher air - fuel ratios in engines will improve fuel efficiencies greatly and lean burn engines can operate efficiently at these ratios. Improvements in lean burn technology, such as stratified charge systems and tumble sprays, help lean burn engines to reach unburned hydrocarbon and carbon monoxide emission targets. However, as engines operate at leaner fuel levels, so nitrogen oxide emissions increase. Therefore, for lean burn engines to replace traditional fuel mixture engines, nitrogen oxide emissions must be reduced. This can be done by catalysts or by recirculation in the engine. However research continues to make lean burn engines ecologically viable. 


Acknowledgements
Orbital engine corporation
NCERQA National centre for environmental resaerch and quality assurance
Sandia technology
Mitsubishi motors
University of Missouri research board


Bibliography
"Advances in 2 stroke cycle engine technology" Selected papers (SAE 1988)
"Emission Control for Spark Ignition Engines" Bosch
"The Internal Combustion Engine in theory and practice" Volume 2 Charles Taylor (MIT Press 1977)
"The future of automotive technology" Seiffert Walter (Francess Pinter 1984)
"Fuel Economy" John C Hilliard + Geaorge S Spinger (Plenum press. 1984) 


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Tumble vs. Swirl
Conventional engines create "swirl" airflow. But this takes the fuel around the outside of the cylinder. It's impossible to concentrate the fuel, so it doesn't always burn completely.
Tumble-shaped airflow solves this problem. The tumble shape breaks up as the cylinder compresses, forming small typhoon-like eddies. These enable fuel to be concentrated around the sparkplug, for outstanding combustion efficiency even with extremely lean air-fuel ratios.
A tumble pattern was employed on the Mitsubishi Vertical Vortex (MVV) lean-burn engine. However, counter-clockwise tumble was not feasible for direct injection.
  
 Why does it have to be clockwise?
If the air tumbles counter-clockwise, it will carry a directly injected fuel spray into the sparkplug, creating a soot buildup that leads to misfires. Also, counter-clockwise flow doesn't allow time for a directly injected spray of petrol to vaporise. These problems were solved by the invention of the Upright Straight Intake Ports, which enabled "clockwise tumble." 
 
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