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Big engines with small engine economy.

Cylinder deactivation – shutting down multiple cylinders during light-load conditions – is becoming increasingly popular as a means of reducing fuel consumption and emissions. We explain how it works.

The idea of shutting down cylinders isn’t new and the best known – unsuccessful – automotive attempt was Cadillac’s ill-fated V8-6-4 effort back in 1980 (photo at right).

The idea of closing off successive cylinders was good, but the implementation was poor; hampered by the limited ability of engine electronic controls at that time.

Since electronics have improved exponentially, Mitsubishi, Mercedes-Benz, Chrysler, Honda, VW, Rolls Royce, GM and Ford have developed engines with various levels of cylinder deactivation (CDA) and more developments are in the pipeline.

In the 4WD world in Australia the first such engine is the 5.7-litre Hemi V8 that was launched in the RAM 1500 in September 2018. This engine uses Chrysler’s multi-displacement system (MDS) to vary operation between eight and four cylinders.



Petrol engines are the obvious candidates for CDA,
because a petrol engine has a throttle plate, to control inlet air volume. The correct air/fuel ratio (AFR) – stoichiometric ratio – is critical in a petrol engine, whereas a diesel engine can operate with excess air.

The throttle plate restricts incoming air when the load on the engine is light, because not much fuel is being injected and the AFR must be maintained by limiting the amount of air getting into the cylinders. That restriction makes the engine work harder, to overcome these ‘pumping losses’.

In addition, engine efficiency drops, because the resulting cylinder pressure is lower than optimal.

By introducing CDA, engine makers can cut out one or more cylinders when engine loads are light, thus eliminating pumping losses and increasing cylinder pressure in those cylinders that are ‘firing’.

Real world economy is improved by between 10-percent and 25-percent.

Another bonus is that exhaust gas temperature is maintained, helping the emissions system in the exhaust to function more effectively.

This latter factor is encouraging research into CDA for diesel engines.Diesels don’t benefit as much as petrol engines from the pumping loss point of view, but can benefit by having exhaust temperature maintained, thus allowing the DPF to function properly.


How CDA works

CDA is done by eliminating valve action, fuel
injection and spark ignition in one or more cylinders. Electronic controls are vital in these functions.

Valve action is eliminated by having an electro-electro-hydraulic ‘link’ in the valve train. In the case of a pushrod engine the hydraulic lifter is modified so that it ‘collapses’ internally when it gets lifted by the cam lobe. The lifter effectively shortens, so that the cam lift isn’t passed on to the pushrod.

In the case of an overhead camshaft engine the actuator or rocker is two-piece and can be ‘separated’ and ‘joined’ by electro-hydraulic action, so that camshaft lift isn’t transferred to the valve stem.

Once activated, CDA shuts down activity in one or more cylinders and the engine rotates as normal, but with a reduced number of ‘working’ cylinders.

Obviously, cylinder cutout is designed into the engine from its inception, so that balance is preserved.

It may seem inefficient to close the valves in a non-operating cylinder, forcing the piston to compress the exhaust gas trapped within. However, that ‘work’ stroke is balanced by the successive ‘expansion’ stroke as the piston falls, so the net result is very little power consumption.

Also, it’s important to isolate that non-working inlet tract, to preserve airflow into the working cylinders and to prevent cool air passing through a non-operating cylinder into the exhaust system and dropping exhaust temperature below the optimal level.


Hemi 5.7 with MDS

Chrysler’s Hemi 5.7 is equipped with CDA that Fiat-Chrysler-RAM-Jeep calls Multi-Displacement System (MDS). This system owes more than a little R&D development to Daimler that had merged with the struggling Chrysler organisation in 1999.

Before this unhappy marriage ended in divorce in 2005 Chrysler accessed not only the platform for the Grand Cherokee, but also insights into Mercedes-Benz’ CDA technology.

The first MDS Hemi 5.7 was released in 2004 and suffered from teething troubles for a few years. A revamped engine was launched in 2009 and that’s the basis of the powerplant in the RAM 1500.

This engine has eight of its 16 lifters able to deactivate valve action and eight normal lifters; a unique camshaft; four solenoids to initiate lifter action and a special exhaust to preserve ‘V8 burble’ when operating on our cylinders.

During MDS operation the same four cylinders are always shut down. They are 1, 4, 6, and 7. (Hemi engine cylinders are numbered from the left front to right rear in the direction of forward travel.)

The firing order is 1-8-4-3-6-5-7–2, so the system shuts down every second cylinder in the firing sequence to maintain rotational balance.

When the engine ECU determines load factors are correct, MDS deactivates the cylinders in firing order within 0.04 seconds, using oil pressure. The MDS solenoids direct oil pressure to the lifters for each cylinder and that pressure pushes in locking pins that allow the lifters to ‘collapse’, so they no longer open the valves. The ECU also shuts off fuel and spark to the cylinders.

If the driver demands more power than four cylinder operation can supply, the MDS lifter ‘lockout’ is disabled in four milliseconds and normal eight-cylinder operation resumes.


How new emissions laws will dictate CD in diesels

CDA is now normal practice for big-bore petrol engines, but it’s also highly likely to be implemented by 2024 in 4WD light-truck diesels, such as those that power 2500 Series yank utes.

The US state of California has been the leader in engine emissions legislation since the 1980s and sooner or later all US sates and most countries – including Australia – have followed that lead. Critics point out the complexity and costs of compliance, often without considering the obvious advantages of modern engines, in terms of reliability, performance and economy.

The latest Californian initiative is a step up from the current US equivalent of Euro VI, while we still have only Euro V in Australia. The August 2020 California Air Resources Board (CARB) initiative requires truck manufacturers to reduce emissions below Euro VI levels for diesel-powered vehicles with 10,000lb (4.5 tonnes) or greater GVM by 2024.

Although CARB has led this latest emissions charge, the United States Environmental Protection Agency (EPA) is also moving to strengthen diesel engine emission standards, with a focus on oxides of nitrogen (NOx) emissions under real driving conditions, including low-speed urban driving.

However, the EPA’s Cleaner Trucks Initiative (CTI) is targeting US-wide implementation somewhat later than in California, from 2027.

Virtually every diesel truck emissions law since the 1980s has been greeted with protest by global engine makers, but almost all of them have managed to comply: Caterpillar being a notable exception. There have been additional costs, but the modern truck diesel performs much better, for longer and with much better fuel economy than pre-emissions engines.

In California, light, medium and heavy trucks – 4.5 tonnes GVM and above – are the largest single source of air pollution from vehicles: responsible for 70 percent of smog-causing pollution and 80 percent of carcinogenic diesel soot, despite the fact that they number only two million among the 30 million registered vehicles in the state.



Because of that situation CARB has set a stringent new reduction of NOx emissions by 90 percent. The new regulation reduces the current heavy-truck NOx standard from 0.20 grams per brake horsepower hour to 0.050 g/bhp-hr from 2024 to 2026 and to 0.020 g/bhp-hr in 2027.

In addition, it reduces permissible heavy-truck particulate matter emissions from the current emission standard of 0.01 g/bhp-hr to 0.005 for 2024 and subsequent model-year engines.

Importantly, the tighter NOx regulations are based on a new ‘low load cycle’ (LLC). This measurement takes into account that diesel engines don’t always operate at high load factors, particularly in urban situations.




Two engine design scenarios

An obvious way to comply with the US CTI program is to enhance existing exhaust after-treatment systems and that’s just what the Manufacturers of Emission Controls Association (MECA) has done.

MECA’s feasibility report was based on engine dynamometer test results and emission models from fully aged after-treatment systems  and concludes that engine and after-treatment technologies are available to achieve certification levels within the 2027 timeframe. 

However, the solutions considered in the MECA assessment are designed to meet the current full useful life of 435,000 miles, so longer emission durability periods still have to be demonstrated.

MECA believes the lower NOx emission standards will be met without sacrificing fuel economy and increasing carbon dioxide (CO2) emissions. Reductions in fuel consumption will be made possible by such engine efficiency technologies as cylinder deactivation, advanced turbochargers and electrification. 


Cummins’ CDA

Cummins Inc and Tula Technology, Inc have collaborated on a demonstration of diesel Dynamic Skip Fire (dDSF). By using dDSF software to control cylinder deactivation, Cummins and Tula have demonstrated significant reductions in emissions and fuel consumption.

“At Cummins, it’s our mission to power a more prosperous world and we do this by helping customers succeed through innovative and dependable products that are good for the customer and the environment,” said Lisa Farrell, director of Advanced System Integration, Cummins Inc.

“Our partnership with Cummins has given us the opportunity to expand our DSF technology beyond its success in gasoline engines,” said R. Scott Bailey, president and CEO of Tula Technology. 

The Cummins-Tula project started in early 2019, using a Cummins X15 Efficiency Series six-cylinder diesel engine. The joint development team modified the engine to use Tula’s DSF control algorithms that command combustion or deactivation per cylinder. 

In the challenging low-load LLC proposed by CARB, the dDSF technology predicted reductions in tailpipe NOx and CO2. The reduction of tailpipe NOx was achieved primarily by optimised exhaust temperature control, resulting in dramatically improved efficiency of the after-treatment system. 

The collaboration is continuing to explore system optimisation and viability to control noise, vibration and harshness in commercial vehicle applications.


Jacobs’ CDA

If there’s one American company that’s well-versed in variable cylinder activation it’s Jacobs. Best known for its famous ‘Jake Brake’ engine braking systems, Jacobs’ history of variable valve timing hardware makes it a ‘natural’ for cylinder deactivation.

Jacobs’ CDA hardware has been developed over the past four years, for multiple heavy duty engine platforms and has been road tested on various vehicles. 

Like the Cummins-Tula initiative, Jacobs’ CDA is said to improve fuel economy by up to 20 percent at low load conditions, while increasing exhaust temperatures for optimal DPF and Selective Catalytic Reduction (SCR) operation.


Daimler CDA

Given the fact that Daimler developed electronically controlled cylinder deactivation for some of its big-bore petrol engines and transferred that technology to Chrysler during the long-abandoned Daimler-Chrysler merger, we thought it obvious that the world’s number one truck maker would be leading the diesel CDA charge.

Daimler may well be doing just that, but our approach for information was met with a terse: “In principle, for competitive reasons, we do not provide any detailed information on technology aspects.”

Although the USA may not be perceived as the free world’s icon these days it’s one of the most influential markets and a leader in emissions legislation. In the transition to electric vehicles the ‘roadmap’ set by CARB’s August 2020 legislation sets a time frame for improvements in internal combustion engine developments. 

What’s clear is that when global markets follow the US lead we’ll see those technologies Down Under.





























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