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Study Finds That Late Intake Valve Closing and Increased Intake Pressure Can Reduce NOx, Soot, HC, and CO Emissions Simultaneously in Diesel PCCI

Summaries of the different effects of late IVC on engine emissions and performance from recent studies. (LIVC: late IVC; ←: advance; ↓decrease; ↑increase; – remain constant). Credit: ACS, Peng and Jia (2009). Click to enlarge.

A study by a pair of researchers from the University of Sussex (UK) and Dalian University of Technology, China found that using late intake valve closing (IVC) in diesel PCCI (premixed charge compression ignition) engines, can, with the assistance of increasing intake pressure, reduce NOx, PM, HC, and CO emissions simultaneously.

A certain EGR (exhaust gas recirculation) rate and optimized SOI (start of injection) timing were always necessary to maintain satisfactory NOx and soot emissions for diesel PCCI combustion. Their paper was published online 14 October in the ACS journal Energy & Fuels.

PCCI in diesel engines offers the potential for substantially reducing NOx and particulate matter simultaneously while maintaining engine efficiency of conventional diesels. Late IVC as a mechanism to optimize PCCI combustion has a complex influence on engine performance and emissions, and those influences can be significantly combined with other operating parameters such as EGR rate, intake pressure, injection timing, and fueling rate.

The low combustion temperature induced by late IVC could help the reduction of NOx emissions and engine noise. However, it also leads to the increase of HC emissions. Late IVC prolongs ignition delay and improves charge homogeneity inside the cylinder,which reduces NOx and soot emissions. On the other hand, since late IVC allows some intake gases to be expelled out the cylinder during the compression stroke, the amount of oxygen available for the combustion is reduced, and this could cause the increase of soot emissions. Higher boost pressure can help to maintain the in-cylinder gas density when late IVC is used, but this increases the compression work and thus possibly leads to higher ISFC. CO emissions have a more complicated behavior for late IVC, and it is sensitive to both combustion temperature and local air-fuel ratio.

—Peng and Jia (2009)

In their study, Zhijun Peng and Ming Jia performed a full engine cycle simulation using a three-dimensional computational fluid dynamics (CFD) model coupled with detailed chemical kinetics to investigate the effect of late intake valve closing (IVC) on combustion and emission characteristics in a PCCI diesel engine.

Specifically, they focused on the potential of late IVC combined with variable intake pressure, EGR rate,and start of injection (SOI) for emissions reduction.

Based on the results, they concluded:

  • Ignition timing could be efficiently controlled by IVC timing because it could change the effective compression ratio.

  • Late IVC avoided high NOx and soot concentrations by keeping lower combustion temperature and providing better premixing. But it probably led to increases of HC and CO emissions due to the lower combustion temperature and insufficient oxygen amount.

  • Increasing intake pressure helped reductions of soot, HC, and CO emissions due to the improved homogeneity of the fuel-air mixture and increased oxygen amount. It is extremely important to increase the intake pressure for soot reduction when late IVC was adopted, they noted. By properly retarding IVC timing and increasing intake pressure simultaneously, all four regulated emission components could be reduced.

  • For diesel PCCI combustion, a certain EGR rate was necessary for maintaining low NOx emission when IVC timing was retarded, although higher EGR rate increased soot emissions almost for all investigated IVC timings.

  • SOI timing should be carefully optimized when variable IVC timing was employed for gaining both low NOx and soot emissions for diesel PCCI combustion.


  • Zhijun Peng and Ming Jia (2009) Full Engine Cycle CFD Investigation of Effects of Variable Intake Valve Closing on Diesel PCCI Combustion and Emissions. Energy Fuels doi: 10.1021/ef900688v



Isn't it amazing that it took only 122 years of ICE manufacturing (after many million engines) to find this out.

Will it take another 122 years to implement these findings?

And many of us would like to see the perfect extended range, failure proof, quick charge, low cost BEV in the first 2 or 3 years?




Isn't "late intake valve closing" what's used in the Atkinson cycle engines that are used in most of the current gas-hybrids?

Roger Pham

Yes. The problem with that in current diesel is that diesel engine already has lower specific power in comparison to homogenous-charged engines. Atkinson cycle will lower the specific power even more, since the effective intake volume is significant less than the exhaust volume. Diesel tractor-trailers are already so under-powered as they are now! To make up for that, they suggest increase the boost pressure, which is well and good, with the requirement that you'll need a larger turbocharger with much larger turbine wheel, since the exhaust gas will be more expanded and have a lot less energy available to turn the compressor, hence, somewhat higher cost, but can be recouped by lower-cost exhaust emission treatment and potentially modest gain in fuel efficiency. The lower energy available in the exhaust gas will also affect engine acceleration and low-end torque, since it will take a lot longer for the turbo to spool up, due to higher spinning mass of the turbo coupled with lower exhaust gas' pressure. Definitely more work will need to be done here to arrive at a drivable and practical engine!

But, the main reason why that was not tried before was that diesel fuel used to be cheap, and exhaust emission standard for diesel vehicle was lax, until recently.


404?DOI Not Found.

......."but this increases the compression work and thus possibly leads to higher ISFC."

I ? Instantaneous sfc specific fuel consumption?Maybe.

This will be all well and good until it needs the services of a repairer.
I keep making this point who the hell is going to fix this stuff.
Reminds me very strongly of all the u beaut anti pollution gear that rarely worked for the warranty period.
Most ground techs with limited resources (and not always the smartest of any class.)
The end up with low fuel consumption, worse emissions, High maintenance costs and little risk of satisfaction.
Maybe highly qualified aircraft engineer types and supervisors that dont get their hands dirty will know which sides up, but the rest of us will
There seems little point in engineering such systems unless you can factor in the ability of service techs to diagnose repair and maintain the same.

Rogers "practical" engine must include serviceability.


Service techs will evolve with various technologies. They always did. Training and re-training programs exist. Secondly, easy to used diagnostic tools will be developed for repair and calibration shops.


Harvey,bear with me for some reason this isnt getting up.

the E motors are all common or garden variety machines ( or will) share electronics and circuits, able to be analyzed by competent service people using generic tools and first principles (with enough time or analytic skills) OR dedicated scan tools and in house dedicated service tools.
These are very expensive and still require a qualified operator trained to make sense of the tool.
A well equipped workshop also requires a thorough knowledge of the associated systems I.E. ice extenders vis capstone turbine, piston , diesel, gasoline, lpg etcc.

By comparison Electric motors with one or two moving parts, that are self evident.


Many of the ice developments require a sense of the history of their evolution (DNA) there are specialist areas: mechanical,dynamics or balancing , hydraulics, electronics and electrical, chemical analysis (5 gas analyser, thermodynamics (exhaust flow and heat sink, boiling, optimising) plus the chassis, dynamics and computers.
I dont see calling these fields by their proper names is an exaggeration.
Few university graduates come out with 1/2 that knowledge let alone expertise.
If that doesn't sound expensive?


Most tradies are lower school leavers.
The manufactures R&D 'teams!' may have a working ability between them.

If you have ever tried to fix something that you do not understand you will know it is only possible if the fault is in a simple and obvious component. A fuse, wire, etc.

In electronics that type of fix can at best be considered temporary and many circuits need first principle understanding to correct the design fault that led to failure.
"why did that fuse blow - not 'replace it,or 'Try a bigger one.
Hence most consumer products are not even serviceable let alone economical to same.


How do you see things changing in the Automotive trade
Just because they are bigger? or more expensive to purchase and therefore a better prospect?
That won't make throw away systems repairable.

Thoughtful and considerate design design will.

If Rogers post went over most of our heads, then I suggest these complexities won't be welcome in many workshops and that's critical to continued performance.

Certification of compliance is required for 'The life of the vehicle' But the life of a 'Product' generically can be 3,6, 12 months or in Kilometers, or till parts are no longer available.

For Toyota in Aus that's ~ 4 years.There are many examples of new machines esp 'thinking motorcycles here' that are not well supported from new and spend the best best years of their lives waiting for parts to keep them going past the first service.


Many parts of the world do not have expertise, workshops or the bucket loads of cash required to maintain these complex systems that is multiple system even on the one manufacturer let alone open to all comers.
Take a look at the number of different vehicles and the complexity that could pass as you stand on any highway anywhere in the world.

We definitely need simple.
Then the tradies will be happy? again with their workplace environment, meet their legal obligations and can continue their often unrecognised services.

Remember most people buy on looks - image?,staying ahead of the neighbours, colour, leather seats, price, then safety, gadgets, size ie bigger = safer,etc and finally ease of service.They dont have to service themselves and what comes out the back is of no concern what so ever.


It was SAAB that was found misleading for advertising the idea that the exhaust from their vehicle as cleaner than that going into the intake.
The advertising agency? knew their market.
As a former trades person in various disciplines, this aspect touches a nerve.
I know the ongoing operation of devices depend entirely on ongoing maintenance and recognition of the problem in the first place preferably 3 months before the first symptoms appear.

Stan Peterson

Harvey D,

Is your entire existence predicated on Conspiracies everywhere? PCCI is a recent advance because only now do we have the adequate means to meter fuel flows and create PCCI mixtures.

Roger, Thanks for your post. But please don't encourage the technically ignorant that some technological advances haven't been made on a pure conspiracy basis. We both know that mnay present technological feats can't be done until the tools and techniques are developed to do so. PCCI and HCCI are relativley ne wunderstnadings of combustion, that weer impossible to measure and understand flong ago; even if they were, the means to accurately control theprocess was lacking.

The great Dr. Langley was unable to fly a mere half decade before the state-of-the-art of the ICE had advanced enough. A few years later a couple of bicycle makers were able to do it, and did. Pity the great Leonardo DaVinci who was half a millenia ahead of the time. He couldn't fly because the tools to make the tools, to make the tools, to make his flying machines hadn't been developed.

Roger Pham

Thanks, Stan, for your input, and for the analogous aviation history which is quite relevant to this situatiion.

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