MCE-5 VCRi: Pushing back the fuel consumption reduction limits

It’s precise
and controls VCR cylinder-by-cylinder

MCE‑5 VCRi provides “effective precision” to compression ratio control that has never been equaled on any other VCR prototype or concept. To understand the basis of this precision, we must remember that through compression ratio control we are trying above all to find a certain type of engine behavior in response to different operating conditions. To this end, compression ratio precision is not an absolute notion but a relative one.

MCE‑5 VCRi controls its compression ratio
cylinder by cylinder. This feature is decisive
for exploiting the full potential of VCR

Cylinder-selective VCR can help with knock
management alongside ignition advance

Compression ignition control requires
the differentiation of each
cylinder's compression ratio

Since it's cylinder selective, MCE‑5 VCRi opens
the way to new strategies for limiting
the exhaust temperature

Each cylinder of the MCE‑5 VCRi is equipped with
a sensor, making it possible to determine the
compression ratio with a precision of 0.1:1

MCE‑5 VCRi cylinder-selective VCR eliminates
dimensional classes for parts while
guaranteeing maximum precision

On a 6-cylinder engine, two deactivated
cylinders at CR 6 instead of CR 10 reduces
fuel consumption by 2 to 3%

For example, though a VCR engine offers high precision in the vertical positioning of its pistons, this positioning does not guarantee that the effective combustion chamber volume will be identical from one cylinder to the next: this volume depends in particular on the precision of the forging and machining of the cylinder head. In the same way, if the volume of each cylinder’s combustion chamber is geometrically identical, there is no guarantee that this is also true for the effective gas compression ratio: this depends on the filling ratio of each cylinder, which itself depends on the geometry of the intake and exhaust pipes and the valve timing. Even if all the cylinders are filled identically, it’s not certain that they’ll behave in the same way with respect to knock for instance. Some cylinders have zones that are more or less hot, others naturally recycle burnt gas or have a blow-by rate that is more or less high.

In the end, what we’re looking for is a behavior rather than a compression ratio value.

To obtain the desired behavior, one must first have sufficient control over the piston altitude to come close to the objective. With MCE‑5 VCRi, it is possible to reach a compression ratio precision of 0.1 points (at high compression ratios), in the worst case. The required corrections must then be made to reach the behavior objective. This leads to one of the essential functions of VCR engines: each cylinder’s compression ratio must be controlled independently.

If, for example, we want to adjust the compression ratio according to knock or the measured in-cylinder pressure, it is necessary to be able to correct each cylinder’s compression ratio independently. This condition is indispensable to obtain the same behavior from all the cylinders, despite their differences. According to this strategy, the geometric compression ratio as defined from the altitude of the piston is only an indicative value: the effective compression ratio at the same piston altitude at TDC is potentially variable according to speed, load and the targeted objectives.

In certain cases, a difference in behavior between cylinders has no real consequences while it is crucial in others.

For example, on fixed compression ratio engines, the knock limit is normally detected by an accelerometer (knock sensor). When knock is detected, it is eliminated cylinder by cylinder by temporarily reducing ignition advance, to then go back up to the knock zone, and so on. In VCR engines, the compression ratio cooperates with the ignition advance to reach this result. Ideally, each cylinder should be independent to be able to choose between reducing the ignition advance or reducing the compression ratio. If the compression ratio is not controlled cylinder by cylinder, the consequences are low and may only result in poorer engine stability (COV: Coefficient Of Variation – NVH: Noise, Vibration, Harshness).

If knock control changes little regardless of whether there is independent cylinder-by-cylinder compression ratio control or not, this is not true for compression auto ignition (CAI: Controlled Auto Ignition – HCCI: Homogeneous Charge Compression Ignition).

The initialization of compression auto ignition depends on different factors. Among these factors are the fuel characteristics of course but also the intake air temperature, the EGR ratio (Exhaust Gas Recirculation), the exhaust gas temperature, variations in the residual burnt gas or in the air/fuel ratio in the volume of the chamber, the oil content in the air, the local temperature of the walls and, of course, the compression ratio. These phenomena triggering combustion are so complex that it is preferable to observe when combustion occurs rather than trying to predict its beginning.

With independent cylinder-by-cylinder compression ratio control, it’s possible to approach the value at which combustion should initiate, to then correct the compression ratio cylinder by cylinder according to what is observed. With this strategy, we can increase the compression ratio of late cylinders, while we lower the compression ratio of cylinders in which combustion is initiated too early. The compression ratio then replaces the spark as used in SI to “time” the combustion according to the angle of the crankshaft. For compression ignition, the enthalpy of the mixture must be controlled, which is easier using the work of the piston than EGR. Moreover, the increase in enthalpy produced by the work of the piston does not depend on the previous cycle, as opposed to exhaust gas temperature: this limits the need to use EGR and marginalizes the “memory” effect that links combustion conditions from one cycle to the next in the same cylinder. We note that correct combustion timing is crucial for limiting noise in CAI, as is the pressure gradient obtained by load stratification.

Another positive point: reducing the use of EGR and “differentiating” the cylinders with the compression ratio considerably simplifies the specifications for controlling the valves. It’s even possible to consider replacing a cylinder-to-cylinder valve control system with simpler systems (such as wide-range electric VVTs), since the cylinder-by-cylinder differentiation is managed by VCR.

Compression ignition is the most obvious field of application for the concept of “effective compression ratio precision” directly implemented in MCE‑5 VCRi technology.

Another point that makes cylinder-by-cylinder compression ratio control important is engine assembly, since VCR engines comprise more parts than conventional engines do. Hence, their chain of dimensions is longer. It is already difficult to guarantee a sufficiently precise compression ratio on a conventional engine, and for a VCR engine, it is even more difficult. This is even truer since on a VCR engine, we must also guarantee the absence of contact between the piston and the cylinder head in case of a failure in the VCR control system.

A solution could consist in “blank mounting” the engine to measure the effective piston altitude and then to rework the piston crown: this type of operation is complicated and uneconomical.

These problems disappear with the MCE‑5 VCRi system: the engine is assembled, the resulting piston altitude is measured, the extreme CR stops are mounted (physical stops in contact with the engine crankcase in case of hydraulic failure) and a “zero point” is given for each cylinder in the control system. The relationship between the effective position of the piston and the position indicated by the position sensor is then memorized. This memory can be updated at any time by allowing the engine to come back into contact with the physical stops, in idle for instance. This is really a purist strategy since the control of the MCE‑5 VCRi compression ratio is linear and behaves like a “swing”. This is not true for multilink engines equipped with cylinder-by-cylinder VCR control: a slight change in TDC between the cylinders is caused by a shift in the position of the moving parts.

All of these characteristics and measures considerably lowers the cost and the complexity of MCE‑5 VCRi assembly.

A last point: on an in-line 6-cylinder MCE‑5 VCRi, it’s possible to consider the deactivation of 2 cylinders depending on certain strategies. In this case, setting these two deactivated cylinders at CR 6 (compression ratio of 6:1) instead of CR 10 can reduce fuel consumption by 2 to 3%.

Cylinder-by-cylinder compression ratio control is one of MCE‑5 VCRi technology’s strong points: this function guarantees the efficiency and the success of VCR.