Increasing the Efficiency of aTwo-Stroke Car Diesel Engine The innovative two-stroke car diesel engine concept from Aumet Oy is based on avery rapid gas exchange through poppet valves in the cylinder head when the piston approaches top dead centre. This rapid gas exchange is achieved by high-pressure scavenging air that is produced externally.

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From 1990 to 2000, many carmanufacturers examined ways ofimproving the two-stroke cardiesel engine. The aim was toproduce a lighter, smaller andmore economical engine. In prin-ciple, a two-cylinder, two-strokeengine with 1litre cylinder dis-placement is equal to a 2-litre,four-cylinder, four-stroke engine.Thus, it would be possible tohalve the number of engine partswhile also significantly reducingweight, volume and productioncosts. However, there were stillproblems that were not solved forthis type of engine, namely, HCemission and the excessive wearof the piston rings.

Project Z EngineIn 1999, Aumet Oy started toresearch a two-stroke car dieselengine called the Z engine, in co-operation with the InternalCombustion Engine Laboratory atHelsinki University of Technology(HUT) and the Energy TechnologyDepartment at LappeenrantaUniversity of Technology (LUT).So far, two Masters theses havebeen completed on the subjectand a third is underway. The firstdealt with the simulation of theprocess and the emissions, whilethe third examined very rapid gasexchange. Modern simulationtools such as Star CD and GT-Power were used in those theses.Aumet’s research project is part ofthe Finnish Engine TechnologyProgramme, ProMotor, and it issupported by the NationalTechnology Agency of Finland,TEKES. There are plans to run atest engine in 2003. For reference,a 1.3-litre, two-cylinder, two-stroke car diesel engine with 100kW output has been designed,Figure 1.

CombinationThe Z engine contains severalnew features. It is a combinationof a four-stroke and a two-strokeengine. The piston pushes theexhaust gases out of the cylinderthrough the exhaust valves in thecylinder head. The high scaveng-ing pressure of the air is producedwith an integrated piston com-pressor. The new charge that iscontrolled by the temperature,

pressure and mass flows entersthe cylinder through the scaveng-ing valves in the cylinder head.There, before fuel is injected, asecondary compression of the airtakes place in the cylinder. Thegeometrical compression ratio ofthe Z engine is between 30–50and the expansion is very long,as in the Atkinson cycle. Thispositively affects the efficiency ofthe engine. As the inlet valves aresmall, it is possible to place thefuel injection nozzle in the mid-dle of the cylinder head and stillhave all the valves parallel withthe cylinder. This enables amonobloc construction to beachieved without a cylinder headgasket. Due to this advantageousconstruction, the cylinder headcan be thinner than in a normaldesign. There is one camshaft inthe Z engine. It rotates at thesame speed as the crankshaft andcan therefore be used as a bal-ancer shaft. The mechanical effi-ciency of the Z engine is high, asthe pistons work at every stroke.According to simulations, theefficiency of the engine was45–48 % with a turbocharger.

Z ProcessA modern 4-cylinder, turbo-charged TDI engine was used asthe standard for comparison andthe Z engine achieved a better fuelconsumption than it. The results ofthis simulation are presented inFigure 2.

The process of the Z engine canuse spark or compression ignition.Internal EGR is easy to implement.

Figure 1: 1.3-litre, two-cylinder, two-stroke car dieselengine with 100 kW.

Ways of improving thetwo-stroke car

diesel engine

THE NEWDIESEL2-STROKECAR ENGINEIS BORN!

– HCC1

– LOW NOX

– HIGH EFFICIENCY

– VERY HIGH PART

LOAD EFFICIENCY

www.aumet.fi

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In normal engines, the peak com-pression pressure and temperatureare tied to the geometrical com-pression ratio of the engine. This isnot the case with the Z engine. It ispossible to separately control thepeak compression pressure andtemperature by using an intercool-er with an ECU-controlled (EngineControl Unit) by-pass valve. Thus,the mass flow over the engine canalso be controlled; for example, ifthe mass flow to the workingcylinder is 1 g/stroke and its tem-perature is 300 K, or the mass flowis 0.5 g/stroke and its temperatureis 600 K, the final compressionpressure stays the same in theworking cylinder. In the latter case,the temperature rise caused by theburning of the fuel is twice ashigh, as the mass of the gas is only50 %. This increases the efficiencyof the engine, especially at partload. In the simulations, the valueof the maximum compressionpressure was 160 bar and the valueof the maximum mean tempera-ture was limited to 1800 K to keepthe NOx formation level low. A 10°shift in the inlet (and exhaust)

cam timing makes it possible tochoose how much compressionoccurs in the cylinder. Theamount of internal EGR is easy tocontrol. The EGR is additionallyused as an "internal heatexchanger”. The rapid expansioncaused by the high compressionratio shortens the high-tempera-ture period by about 50 %. Thisinfluences the NOx formationand the heat losses, Figure 3. Theprocess parameters are shown inthe simulation results in Figure 4.

Z CombustionThe high scavenging pressureenables this new combustionmethod to be implemented in theZ engine. The shape of the com-bustion chamber is toroidal and itis located on the piston crown.The maximum speed of the gascoming into the cylinder is above500 m/s. The pressure of the gasvaries between 7–25 bar. The swirlnumber of the new charge is about10. The turbulence energy is veryhigh, about ten times higher thanthat of normal diesel engines.

Figure 2: Simulation results.

Figure 3: NOx formation and heat losses.

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The high turbulence energymakes it possible to mix thecharge effectively as well as tokeep the temperature differencebetween the burned and theunburned charge small, thusenabling rapid and soot-freecombustion. The fuel injectordoes not need to have any trans-port function in the Z combustionprocess as the high-velocity airtakes care of the transportation ofthe droplets. All the air passes thefuel injector when the air rotatesin the groove on the pistoncrown. This makes a controlleddistribution of the fuel to the airpossible, as in an oil burner. Forthis reason, it is possible to opti-mise the atomisation of the noz-zle. The nozzle produces verysmall droplets. The size is aboutone micron. The nozzle producesa homogeneous mixture with thelambda, ignition and ROHR con-trols. The droplets evaporate andform a homogenous mixture withthe air before they burn. No fuelinjection occurs to the flame inthe Z combustion process and theamount of particulates is there-fore very low, Figure 5. Thedesign of the nozzle means that ithas no sac volume, thus loweringHC emissions. The fuel injectorcan, for example, be like the CAVMicrojector, which was used insome USA-made swirl chamberdiesel engines in the 80s.

Valve MechanismThe gas exchange of the Z enginetakes place during a period of20–30° crank angle. This meansthat, when the engine rotates at3600 rpm, the gas exchange takesplace within a period of onlyabout one millisecond. The maxi-mum acceleration of the inletvalve is 4000 g, which means thateach gram in the valve line pro-duces a force of 40 N. In order tokeep the contact stress on thecam within acceptable limits, theinlet cam has a basic diameter of100 mm. The Hertzian contactpressure is about 1200 N/mm_.The material of the cam followerroll is silicone nitride and thecam is through-hardened. In thiscase, the maximum allowablecontact pressure is 2500 N/mm2.The lift of the inlet valve is small,only 3.6 mm, and the valve

spring is a combination of amechanical spring to start theengine and a pneumatic spring torun it. The force of the pneumat-ic spring can be adjusted depend-ing on the speed of the engine.The behaviour of the valve mech-anism is simulated with a valvetrain simulation programme. Thedata of the valve system is shownin Figure 6a and 6b.

CompressorAn adjustable turbocharger (or asupercharger) is used as a pre-compressor. It defines the massflow over the engine. A piston-type compressor is used to raisethe scavenging pressure to 20–25bar. The maximum pressure ratioof the piston compressor can be10 when using a typical compres-sor head design. The drive for thepiston compressor is taken fromone connecting rod of the engine.The side force of the compressorpiston is small, due to the veryelliptical movement of the lowerend of the connecting rod of thecompressor piston. The force ofthe compressor piston is only10–15 % of the force of the work-ing piston, thus allowing thecompressor to be made lighter.The cost of the integrated com-pressor is about 30 % of the costof one working cylinder. There isan intercooler with a controllableby-pass valve after the pistoncompressor. The size of the inter-cooler can be small, as the pres-sure of the compressed air is high.

ConclusionBy using the Z engine, it is possi-ble to reduce the manufacturingcosts of vehicles. All the compo-nents used in the Z engine arelike those used in normal enginesand compressors. For this reason,there is no need to make manychanges to the component supplychain. It is possible to have adiesel car without an NOx cata-lyst or a particulate filter whenusing the Z engine.

by M.Sc. Timo Janhunen, Aumet Oyand Prof. Martti Larmi, HelsinkiUniversity of Technology.

Figure 6a + 6b: Data of the valve system.

Figure 4: Process parameters.

Figure 5: Z-process.