About

Wai K. Cheng is a Professor of Mechanical Engineering at the Massachusetts Institute of Technology (MIT). His research interests include engine performance and emissions, combustion science related to engine behavior, novel energy conversion methods, and the impact of transportation systems on quality of life and society. He has held various academic positions at MIT since 1980, including Assistant Professor, Associate Professor, and Professor, and has served as the Director of the Sloan Automotive Lab since 2009. Cheng's educational background includes a B.Sc. from California Institute of Technology in 1974, an M.Sc. from MIT in 1975, and a Ph.D. from MIT in 1979. His professional contributions encompass significant research in engine combustion, emissions reduction, and energy conversion, with numerous publications and patents. He has been recognized with awards such as the Society of Automotive Engineers Oral Presentation Award and has served on editorial boards and various committees related to automotive engineering and transportation.

Research topics

• Automotive engineering
• Environmental science
• Materials science
• Computer science
• Waste management

Selected publications

• Performance Assessment of Extended Stroke Spark Ignition Engine

  SAE technical papers on CD-ROM/SAE technical paper series · 2018-04-03 · 5 citations

  articleSenior author

  <div class="section abstract"><div class="htmlview paragraph">The performance of an extended stroke spark ignition engine has been assessed by cycle simulation. The base engine is a modern turbo-charged 4-stroke passenger car spark-ignition engine with 10:1 compression ratio. A complex crank mechanism is used so that the intake stroke remains the same while the expansion-to-intake stroke ratio (SR) is varied by changing the crank geometry. The study is limited to the thermodynamic aspect of the extended stroke; the changes in friction, combustion characteristic, and other factors are not included. When the combustion is not knock limited, an efficiency gain of more than 10 percent is obtained for SR = 1.5. At low load, however, there is an efficiency lost due to over-expansion. At the same NIMEP, the extended stroke renders the engine more resistant to knock. At SR of 1.8, the engine is free from knock up to 14 bar NIMEP at 2000 rpm. Under knocking condition, the required spark retard to prevent knocking is less with the extended stroke. Then the operating point is closer to that of most efficient timing and the efficiency penalty due to knock constraint is reduced. With the extended stroke, since less exhaust energy is delivered to the turbine, the engine air throughput and thus the output power is reduced. At low speeds, the increase in efficiency overpowers the decrease in air flow so that the maximum NIMEP at a fixed speed increases with SR. At high speed, however, the reverse is true and the maximum NIMEP decreases with SR. For the engine/turbocharger combination used in this study, the transition point is at approximately 1500 rpm.</div></div>

  PublisherDOI

• A study of soot formation in a rapid compression machine at conditions representative of cold-fast-idle in spark ignition engines

  International Journal of Engine Research · 2018-05-28 · 5 citations

  articleOpen accessSenior author

  The soot yield, defined as the ratio of the soot mass to the carbon mass in the fuel, for the homogeneous combustion of a rich fuel-air mixture has been measured in a rapid compression machine using the laser light extinction method. The temperature and pressure conditions are representative of those in spark-ignition direct-injection engines at cold-fast-idle. The fuels used are a certification gasoline (with 28% aromatic content) and a blend of the gasoline with toluene (the blend had 40% aromatic content by volume) so that the sensitivity of soot formation to the fuel aromatic content could be assessed. Beyond a threshold fuel equivalence ratio (ϕ) value, the soot yield increases exponentially with ϕ. The soot yield of the gasoline–toluene blend is four to six times higher than that of the gasoline. The soot yield decreases exponentially with temperature, by a factor of 0.58 for every 10 K increase in temperature. In the 657–695 K temperature range, the threshold ϕ value increases linearly from approximately 2.4 to 2.7, at a rate of 0.1 point per 10 K rise in temperature. This temperature dependence is insensitive to the charge density.

  PublisherOA PDFDOI

• Potential of Negative Valve Overlap for Part-Load Efficiency Improvement in Gasoline Engines

  SAE International Journal of Engines · 2018-04-03 · 9 citations

  articleOpen accessSenior author

  <div class="section abstract"><div class="htmlview paragraph">This article reports on the potential of negative valve overlap (NVO) for improving the net indicated thermal efficiency (η <sub>NIMEP</sub>) of gasoline engines during part load. Three fixed fuel flow rates, resulting in indicated mean effective pressures of up to 6 bar, were investigated. At low load, NVO significantly reduces the pumping loses during the gas exchange loop, achieving up to 7% improvement in indicated efficiency compared to the baseline. Similar efficiency improvements are achieved by positive valve overlap (PVO), with the disadvantage of worse combustion stability from a higher residual gas fraction (x<sub>r</sub>). As the load increases, achieving the wide-open throttle limit, the benefits of NVO for reducing the pumping losses diminish, while the blowdown losses from early exhaust valve opening (EVO) increase. However, a symmetric NVO strategy combined with a shorter exhaust duration has a higher potential for reduction in part-load fuel consumption, as the EVO timing can be optimized to minimize the blowdown losses.</div></div>

  PublisherOA PDFDOI

• Back pressure effect on three-way catalyst light-off

  International Journal of Engine Research · 2018-06-04 · 8 citations

  articleSenior authorCorresponding

  The effect of back pressure on the light-off of a modern spark ignition engine three-way catalyst has been assessed by measuring the hydrocarbon conversion efficiency in a hot flow bench and in the cold-idle period in an engine. In the flow bench experiment, a small amount of propane/air mixture is used as a surrogate for the hydrocarbon mixture. The conversion efficiency is found to be only a function of temperature. The efficiency is independent of pressure, space velocity, and the equivalence ratio of the hydrocarbon mixture for λ ≥ 1. In the engine test, while the engine-out exhaust gas temperature is higher at a higher back pressure, there is little difference between the gas temperatures at the catalyst entrance for different back pressures at retarded spark timing. This observation is attributed to the larger amount of exhaust hydrocarbon conversion oxidation between the engine exit and the catalyst entrance with the lower back pressure. The heat release from this oxidation compensates for the lower engine-out exhaust temperature at the lower back pressure. The catalyst temperature increases modestly and light-off time shortens correspondingly at the higher back pressure. This observation is attributed solely to the increase in mass flow rate (and thus exhaust sensible enthalpy flow rate) of the engine needed to overcome the additional pumping loss due to the throttling of the exhaust. These results have been confirmed with a simple one-dimensional catalyst model.

  PublisherDOI

• Effects of Ethanol Evaporative Cooling on Particulate Number Emissions in GDI Engines

  SAE technical papers on CD-ROM/SAE technical paper series · 2018-04-03 · 27 citations

  articleOpen accessSenior author

  <div class="section abstract"><div class="htmlview paragraph">The spark ignition engine particulate number (PN) emissions have been correlated to a particulate matter index (PMI) in the literature. The PMI value addresses the fuel effect on PN emission through the individual fuel species reactivity and vapor pressure. The latter quantity is used to account for the propensity of the non-volatile fuel components to survive to the later part of the combustion event as wall liquid films, which serve as sources for particulate emission. The PMI, however, does not encompass the suppression of vaporization by the evaporative cooling of fuel components, such as ethanol, that have high latent heat of vaporization. This paper assesses this evaporative cooling effect on PN emissions by measurements in a GDI engine operating with a base gasoline which does not contain oxygenate, with a blend of the gasoline and ethanol, and with a blend of the gasoline, ethanol, and a hydrocarbon additive so that the blend has the same PMI as the original gasoline. As such, the dilution and the evaporative cooling effects of the ethanol could be separated. Measurements have also been done with methanol and MTBE. The results show that evaporative cooling effect can significantly change the PN emission. The extent of the change, however, depends on the details of the operating condition such as injection timing, engine coolant temperature, and load.</div></div>

  PublisherOA PDFDOI

• Assessment of Gasoline Direct Injection Engine Cold Start Particulate Emission Sources

  SAE International Journal of Engines · 2017-03-28 · 17 citations

  articleOpen accessSenior author

  <div class="section abstract"><div class="htmlview paragraph">The gasoline direct injection (GDI) engine particulate emission sources are assessed under cold start conditions: the fast idle and speed/load combinations representative of the 1<sup>st</sup> acceleration in the US FTP. The focus is on the accumulation mode particle number (PN) emission. The sources are non-fuel, combustion of the premixed charge, and liquid fuel film. The non-fuel emissions are measured by operating the engine with premixed methane/air or hydrogen/air. Then the PN level is substantially lower than what is obtained with normal GDI operation; thus non-fuel contribution to PN is small. When operating with stoichiometric premixed gasoline/air, the PN level is comparable to the non-fuel level; thus premixed-stoichiometric mixture combustion does not significantly generate particulates. For fuel rich premixed gasoline/air, PN increases dramatically when lambda is less than 0.7 to 0.8. This lambda threshold does not correlate with engine speed and load; however, it increases slightly when the intake air is diluted with nitrogen. To assess the fraction of the GDI injected fuel that contributes to the PN, at fixed load and lambda, the injected gasoline is systematically reduced, with premixed methane as balance for the fuel. It is found that only a small fraction of the injected fuel contributes to the PM emissions. The fraction is dependent on the fuel amount and the rigor of the mixing process; hence it is dependent on speed, load, and timing of the injection.</div></div>

  PublisherOA PDFDOI

• Back Pressure Effect on Three-Way Catalyst Light-Off

  The Proceedings of the International symposium on diagnostics and modeling of combustion in internal combustion engines · 2017-01-01

  articleOpen accessSenior author

  The effect of back pressure on the light-off of a modern spark ignition engine 3-way catalyst has been assessed by measuring the hydrocarbon (HC) conversion efficiency in a hot flow bench and in the cold-idle period in an engine. In the flow bench experiment, small amount of propane/air mixture is used as a surrogate for the hydrocarbon mixture. The conversion efficiency is found to be only a function of temperature. The efficiency is independent of pressure, space velocity, and the equivalence ratio of the hydrocarbon mixture for λ 1. In the engine test, while the engine-out exhaust gas temperature is higher at a higher back pressure, there is little difference between the gas temperatures at the catalyst entrance for different back pressures at retarded spark timing. This observation is attributed to the larger amount of exhaust HC oxidation between the engine exit and the catalyst entrance with the lower back pressure. The heat release from this oxidation compensates for the lower engine-out exhaust temperature at the lower back pressure. The catalyst temperature increases modestly and light-off time shortens correspondingly at the higher back pressure. This observation is attributed solely to the increase in mass flow rate (and thus exhaust sensible enthalpy flow rate) of the engine needed to overcome the additional pumping loss due to the throttling of the exhaust. These results have been confirmed with a simple 1D catalyst model.

  PublisherDOI

• Lubricant Formulations to Enhance Engine Efficiency in Modern Internal Combustion Engines

  2017-04-19 · 1 citations

  reportOpen access1st authorCorresponding

  The research program presented aimed to investigate, develop, and demonstrate low-friction, environmentally-friendly and commercially-feasible lubricant formulations that would significantly improve the mechanical efficiency of modern engines without incurring increased wear, emissions or deterioration of the emission-aftertreatment system.

  PublisherOA PDFDOI

• Development of a reduced chemical mechanism targeted for a 5-component gasoline surrogate: A case study on the heat release nature in a GCI engine

  Combustion and Flame · 2017-02-07 · 45 citations

  articleOpen access
  PublisherOA PDFDOI

• Analysis of NOx Emissions during Crank-Start and Cold Fast-Idle in a GDI Engine

  SAE International Journal of Engines · 2017-03-28 · 24 citations

  articleOpen accessSenior author

  <div class="section abstract"><div class="htmlview paragraph">The NOx emissions during the crank-start and cold fast-idle phases of a GDI engine are analyzed in detail. The NOx emissions of the first 3 firing cycles are studied under a wide set of parameters including the mass of fuel injected, start of injection, and ignition timing. The results show a strong dependence of the NOx emissions with injection timing; they are significantly reduced as the mixture is stratified. The impact of different valve timings on crank-start NOx emissions was analyzed. Late intake and early exhaust timings show similar potential for NOx reduction; 26-30% lower than the baseline. The combined strategy, resulting in a large symmetric negative valve overlap, shows the greatest reduction; 59% lower than the baseline. The cold fast-idle NOx emissions were studied under different equivalence ratios, injection strategies, combustion phasing, and valve timings. Slightly lean air-fuel mixtures result in a significant reduction of NOx. The engine-out emissions are highly sensitivity to combustion phasing. Initial retard results in lower mass NOx emissions up to CA50 around 40 °CA aTDC-compression. After this point the increase in fuel consumption and the reduction on residual fraction reverse the trend leading to an increase in NOx mass emissions for later combustion phasing. Moderate exhaust timing advance is beneficial for NOx emissions and has no impact on the exhaust enthalpy flow.</div></div>

  PublisherOA PDFDOI

Recent grants

• Collaborative Research: NSF/DOE Partnership on Advanced Combustion Engines: Advancing Low Temperature Combustion and Lean Burning Engines for Light- and Heavy-Duty Vehicles with Mi

  NSF · $580k · 2013–2016

Frequent coauthors

• John B. Heywood

  12 shared

• Kevin Cedrone

  C-2 Innovations (United States)

  7 shared

• J. Felipe Rodriguez

  6 shared

• Kenneth Kar

  Concawe

  6 shared

• Dave K. Verma

  McMaster University

  4 shared

• Nick Collings

  University of Cambridge

  4 shared

• Halim Santoso

  4 shared

• Angela J. Acocella

  Engineering Systems (United States)

  4 shared

Labs

• Wai K Cheng LabPI

Awards & honors

• Society of Automotive Engineers Oral Presentation Award (200…
• Society of Automotive Engineers Oral Presentation Award (200…
• Fellow, Society of Automotive Engineers (2003)
• Carl Richard Soderberg Professorship in Power and Propulsion…
• Lester Gardner Fellow (1975)