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

It reduces friction

VCR engines often generate higher friction losses than conventional engines. This is due to the fact that they are more complex, often have more moving parts and require new means to manage the compression ratio. This increase in friction losses is made even more critical by the fact that VCR’s primary objective is downsizing, which leads to stronger forces applied to the mechanical parts. On a conventional engine, an increase in specific power and torque (kW/L, Nm/L) leads to bigger moving parts that generate higher friction losses. Planning high specific power and torque in a VCR engine leads to the same results, which must be minimized in an even more constraining mechanical context.

The FMEP / (kW/L) ratio is one of the strong
points of MCE‑5 VCRi technology

The MCE‑5 VCRi gear system has a high
mechanical efficiency, of about 0.997

The MCE‑5 VCRi piston is no longer subjected
to radial stress:this helps maintain FMEP
at the lowest possible level

The thermodynamic benefit of CAI can be
ruined by excessive friction losses

MCE‑5 VCRi reduces the contribution of pistons
to FMEP but adds that of gears

The relationship between mechanical load and friction losses is well illustrated by Diesel engines. This type of engine produces high FMEP (Friction Mean Effective Pressure), which is due to the very high specific torque of modern Diesel engines (up to 2829 bar of BMEP) and the high maximum pressures reached in their combustion chambers, roughly 160 to 180 bar, and even 200 bar for the latest generations of car Diesel engines.

In order to strongly reduce fuel consumption, a VCR engine must also be subject to high in-cylinder pressures, in the range of 120 to 130 bar. It must also reach extremely high specific torques coming from roughly 35 to 40 bar of BMEP, with extreme powers per liter of approximately 120 to 150kW/L. It’s only in this way that VCR will be efficient enough to successfully compete with the next generations of conventional GDI turbo engines. Nevertheless, a dilemma remains: how to avoid high increases in FMEP because the engine has a variable compression ratio? If this is not possible, the energy efficiency gains will likely disappear.

FMEP is a relative notion. For example, the most modern downsized engines have a higher FMEP than do ordinary engines and yet they offer better efficiency during standard use. In the end, it’s the FMEP/(kW/L) ratio that is the most important. This ratio must be as low as possible, regardless of the type of engine, downsized or not.

Based on the formula FMEP/(kW/L), if we double the FMEP while doubling the engine’s power per liter, the total friction losses of the driving cycle remain identical. It’s easy to understand that the aim is to increase the engine’s specific power (kW/L) faster than its specific friction (FMEP). If this is the case, downsizing is accompanied by a reduction in energy losses due to friction and also by a reduction in pumping losses (PMEP = Pumping Mean Effective Pressure).

An engine’s specific power (kW/L) is not the only factor weighing on friction performance; its specific torque at low speeds (Nm/L) also plays an important role. We find ourselves with a dilemma: on one hand, increasing torque at low speeds increases the size of the engine’s moving parts, which increases its friction losses. On the other hand, high torque at low speeds reduces the engine’s operating speed (downspeeding), which reduces friction and pumping losses (FMEP, PMEP). It all boils down to finding the right balance.

Downspeeding can potentially be very effective in reducing friction losses, as an engine’s FMEP increases more than proportionally to engine speed. Engine downspeeding provides several advantages: the FMEP is lower with a lower rpm, the number of rotations is lower with the same service provided and the IMEP/FMEP ratio is higher (Indicated Mean Effective Pressure/ Friction Mean Effective Pressure). What’s more pumping losses are reduced when the engine runs at lower speed since the engine is more highly loaded.

An important point concerning hard downsized and downspeeded high-torque engines is the sensitivity of FMEP to load. We generally observe that load only has a small influence on the level of engine friction, much less than engine speed does. This is true so long as we stay under a BMEP of 12 to 15 bar. Beyond that, it is no longer true. Yet, high-load VCR engines must reach a BMEP of 35 to 40 bar. At these levels of load, the FMEP can become very high in conventional engines, creating a non-negligible absolute energy loss, even if the relative loss is low (high IMEP/FMEP ratio).

Many VCR engines concepts have increased friction losses. For example, VCR engines based on multiple conrods have an FMEP that increases overly quickly with load. For this type of VCR technology, the FMEP shoots up as of only 4 to 5 bar of BMEP. This renders VCR useless since its thermodynamic gains are partially or totally lost in mechanical friction.

VCR is itself a puzzle: how to make a complex high-loaded engine a low-friction one? Though decisive, additional gains from VCR remain moderate compared with those of conventional turbo GDI VVL engines that should be produced in the next ten years (25 bar of BMEP and 120 kW/L). These gains will be in the order of 5 to 20% depending on the targeted power. If CAI (Controlled Auto Ignition) is added, an additional 5 to 15% will be available depending on the vehicles. These very attractive gains will be quickly challenged if there are overly-high friction losses.

A lot of VCR engines have fallen into this trap: to control VCR and bear high loads, the engine’s moving parts must be bigger and more complex, which will increase the engine’s FMEP. In this case, a choice will have to be made between two strategies that are equally useless. The first consists in avoiding loading the engine to limit friction: this wipes out the fuel consumption reduction made through hard downsizing-downspeeding, which represents the main benefit of VCR, other than CAI. The second strategy consists in loading the engine to take advantage of hard downsizing while sizing the moving parts accordingly. In this case, the gains generated by reducing the cubic capacity are reduced and even eliminated by excessive additional friction. The only remaining solution for the engine is CAI, which is only worthwhile in the single case where the FMEP/(KW/L) ratio remains at least as low as that of a conventional engine, otherwise the CAI gain will be fully or partially lost in friction.

In this context, MCE‑5 VCRi offers characteristics that are unique and decisive. Firstly, an extremely low FMEP/(kW/L) ratio, which is optimized by both factors simultaneously: low FMEP and high specific torque. This favorable ratio is due to moving parts with dissociated functions: the piston receives the gas thrust but it is protected from the radial stress induced by the conrod. The friction between the piston and the cylinder is thus strongly decreased, as is the stress applied to the cylinder. Moreover, the MCE‑5 roller bearing and gear system is not sensitive to the obliquity of the conrod that is assumed by a pure rolling device, nor is it sensitive to crank angle phasing of max in-cylinder pressure. In addition, the MCE‑5 crankshaft has a big overlap that easily assumes high-amplitude torsion stress without needing large-diameter bearings. The end result is that the MCE‑5 gear system has an efficiency so high (0.997) that the additional friction losses that it generates are lower than those they permit to avoid at the contact between the piston and the liner. Remember that the piston, ring pack and liner assembly represents between 40 and 60% of the total FMEP of conventional engines.

Furthermore, the MCE‑5 engine’s FMEP increases very little with load. However, it does increase quite quickly with engine speed (over 3500 rpm). This is perfect for a downsizeddownspeeded engine. At low engine speeds, the MCE‑5 VCRi has exceptionally low friction given its specific power.

MCE‑5 VCRi’s technology eliminates VCR’s dilemma between complexity, load capacity and FMEP, and what’s more, this technology has all the functions required to ideally serve compression ignition: quick, accurate and widerange cylinder-selective VCR control.

Slight differences in FMEP lead to significant variations in fuel consumption. Hard-downsized
engines remain less sensitive to these differences

The efficiency of the MCE‑5 VCRi gear system has a strong influence on final FMEP.
Its present efficiency is 0.997