The Z combustion System

See also:

• The high Swirl Scavenging Valves
• Some 3-d simulation results for Z combustion
• A photo of the prototype engine piston with Z combustion chamber

Timo Janhunen has developed the Z combustion system with local ignition and air to fuel ratio control. The controlled high velocity swirl in the Z motor enables the use of the Z combustion. The combustion chamber is a circular groove in the piston. The combustion air flows in the combustion chamber at top dead center. The fuel is injected down stream to the highly turbulent air flow. The squish flow turns the swirl flow to screw type flow in the chamber (see the picture). This makes better mixing for its part.

The combustion time is shorter than in common diesel engines, because of the very high turbulence and better mixing. This makes a positive affect to the efficiency of the engine.

Because of the high swirl, the fuel injector does not need transport function in the Z combustion system. For example, the CAV Microjector type injectors could be used. To learn more about Microjector –type injectors see:

• MTZ Motortechnische Zeitschrift 41 (1980), number 3, page 90
  
• MTZ Motortechnische Zeitschrift 42 (1981), number 2, page 47
  
• Cornel Stan: Direkteinspritzsysteme: für Otto– und Dieselmotoren. Springer – Verlag, 1999. ISBN 3-540-65287-6
• Xu Min, Lee E. Markle, Delphi Automotive Systems: CFD-Aided Developement of Spray for an Outwardly Opening Direct Injection Gasoline Injector. SAE Paper 980493
• Howes Peter, Lucas CAV Ltd.: The New Cav Microjector Injector. SAE Paper 800509

Near Stoichiometric Homogeneous Combustion in the Z combustion System

It is possible to use near stoichiometric homogeneous combustion in the Z combustion system. To achieve homogenous combustion, the fuel must be vaporized and mixed completely before the ignition. Stoichiometric combustion minimizes the air consumption. The NO_x and the particle formations are assumed to be low in this kind of combustion. The Z combustion system has an analogy with the swirl chamber combustion without the known disadvantages (heat losses etc.).

The homogeneous mixture burns in the combustion chamber in equivalence ratio 0.8. There is not enough oxygen in the chamber. Most of the hydrogen of fuel burns to water (H₂O) and most of the carbon oxides to carbon monoxide (CO). When the piston goes down, the unburned carbon monoxide mixes with the air in the gap between the cylinder head and piston crown. While mixing, the carbon monoxide burns to carbon dioxide (CO₂). Similar phenomenon is well known in pre and swirl chamber combustion.

It is possible to use radial / tangential grooves on the piston grown to affect mixing while piston goes down and partially unburned gas from the combustion chamber meets the air in the gap between cylinder head and piston grown. The grooves can be quite small so that they do not degrease the swirl. It is also possible to use an additional air chamber in the piston or in the cylinder head to increase mixing.

In order to understand the soot and NO_x free homogeneus combustion, see: Merker, Günter P. Stiesch, Gunnar. Technische Verbrennung. Motorische Verbrennung. Stuttgart, Leipzig, Teubner, 1999. ISBN 3-519-06381-6 pages: 82, 83, 104, 114, and 124

Measured effects of EGR and equivalence ratio to NO –formation in homogenous combustion are explained in the source on the page 124 (picture). According to the source, the maximum NO –emission at EGR rate of 20% is 2 g/kWh. Correspondingly, at 0% EGR the NO –emission is 16 g/kWh and at 10% EGR the NO emission is 6.5 g/kWh.

The EGR rate can be kept in 20% in the Z combustion to minimize the NO_x emissions.

In the homogenous Z combustion, the total equivalence ratio can be kept near 1.0 after the second phase of the combustion. In the second phase, CO burns to CO₂ consuming the air in the gab between piston and cylinder head. In this case, it is possible (if needed) to use a cost effective 3-way catalyst like in spark ignition engines.

More related information: Kahrsted J. Manns J. Sommer A. Wormbs T. Brennverfahrensseitige Ansatzpunke für Pkw-Dieselmotoren zur Erfüllung künftger Eu- und US-Abgasstandards (Combustion Process Improvements for Passenger Car Diesel Engines to fulfill Future European and US Legislations). IAV Gmbh. in 5. Internationales Stuttgarter Symposium, Kraftfahrwesen und Verbrennungsmotoren, Expert Verlag, 2003

The Z combustion can be used to minimize particle emissions with the biofuels like rape oil. This is due to the small size of the droplets and the high turbulence level combined with the technology, where the fuel is not injected directly into the flame. For related information concerning bio fuels, see: Mollenhauer, Klaus, Handbuch Dieselmotoren, Springer-Verlag 2002. ISBN 3-540-41239-5 pages: 163, 166