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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.

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