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

Pushing back the limits of efficiency

The limits of fixed compression ratio internal combustion engines will soon be reached. With MCE‑5 VCRi, the compression ratio becomes variable to push performance and efficiency limits back as far as possible. A new element of the car engine technological mix, MCE‑5 VCRi will “go after” these last unattainable grams of CO2/km at an attractive price and reduce the cost of reaching high-energy efficiency.

There are only two strategies to reduce IC engine fuel consumption: improving the BSFC map (Brake Specific Fuel Consumption g/kWh) particularly in the most used zones of the driving cycle, and resituating the main operating points at better BSFC points, in relation to the driving cycle:

There are only two strategies to reduce IC engine fuel consumption

The first strategy (passage from figure 1 to 2) is based on both the optimization of the effective gas expansion ratio and on the reduction of heat, pumping and friction losses, and of unburnt gases.

The second strategy (passage from figure 2 to 3) is mainly based on the increase in the engine’s specific torque and power (Nm/L – kW/L), to reduce its cubic capacity (downsizing) or mean operating speed (downspeeding).

As an alternative to downsizing-downspeeding, the second strategy can use hybridization, which consists in the temporary storage of energy via electrical means (generator --> batteries --> electric motor). Though it is efficient, hybridization is nevertheless expensive, which is an obstacle to its dissemination.

Though the advantages of these two strategies can’t simply be added up (passage from figure 1 to 2, then 2 to 3), they are combinable. These two strategies aim at improving the positive work (increase in thermodynamic efficiency) and reducing the negative work (increase in effective efficiency).

A PV diagram (pressure volume) shows the 5 levers
that can be used to increase positive work
while decreasing negative work

This balance between positive work and negative work is shown on the PV diagram (pressure volume) in figure 4. We can distinguish 5 characteristic points on this diagram, which are: the increase in the effective gas expansion ratio in the high pressure area (1) or low pressure area (3) of the thermodynamic cycle, reduction in heat losses (2), reduction in pumping losses (4) and reduction in friction losses (5).

MCE‑5 VCRi is appropriate for both of these essential strategies (passage from figure 1 to 2, then 2 to 3), and for each of the points identified on the PV diagram (figure 4). Indeed, the optimization of the effective gas expansion ratio is directly served by VCR, the reduction in pumping losses in served by downsizing, downspeeding and CAI-HCCI (dethrottling), and the heat losses are reduced by CAI-HCCI. Finally, the friction losses are reduced by downsizing and by the MCE‑5 VCRi gear system.

MCE‑5 VCRi can advantageously be combined with other technologies (turbocharging, VVT, VVL, GDI) to reach the best result. It’s important to note that VCR becomes meaningless if used without turbocharging.

Similarly to the diagram in figure 4, these tables demonstrate that VCR has better control
over the parameters that are decisive for engine efficiency (strategies and technologies aiming
to improve effective engine efficiency over the driving cycle)

MCE‑5 VCRi is continuing the course of automotive history: each time a new technology was discovered, it brought new advantages and progress. Engine improvement has followed a long asymptote, made up of sub-asymptotes that are each based on a new technology providing new advances:

As is shown on the graph here above, the widespread use of 4-valves per cylinder and VVTs enabled a reduction in engine cubic capacities (downsizing) to reduce their fuel consumption. GDI coupled with turbocharging and VVL (Variable Valve Lift) went even farther in downsizing and downspeeding, as well as in pumping loss reductions. The last step, beyond which there will likely be no other opportunities, is VCR. VCR will provide very appreciable gains compared with future conventional VVL turbo GDI engines with high performance-density, despite VCR being located on the flat part of the efficiency improvement asymptote.

VCR will provide a significant increase in energy efficiency beyond what existing technologies may provide.
Above: 1.3 ton - 120 kW vehicles. VCR reduces fuel consumption by 11% (proven)
and up to 21% (remains to be proven using CAI) compared with the future
high-loaded GDI turbo engines that will likely be marketed in the coming decade

The first benefit of VCR is that it makes it possible to intensively continue increasing specific performance, as has been the case since the 1900’s, leading to a gradual reduction in cubic capacity for the same service rendered (downsizing), with in addition a noteworthy reduction in energy consumption:

Downsizing is far from new: it has existed since the beginnings of the automobile

Historically, most of the progress made on internal combustion engines
was made possible by new technologies

MCE‑5 VCRi will thus push back the efficiency limits accessible to conventional engines. It will become part of the current technological mix to optimize energy efficiency, adapt the absolute cost of vehicles and reach cost/benefit objectives.

MCE‑5 VCRi will push back the efficiency limits available to conventional engines