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

It only applies to high-end vehicles

Due to an expensive after-treatment system,
Diesel engines will be increasingly reserved
for powerful top-of-the-range vehicles

It has always been more difficult to reduce the fuel consumption of small, low-end cars than it has been to reduce the fuel consumption of powerful high-end, gas-guzzling cars. It’s true that downsizing a small engine that has been sized according to the vehicle’s real needs is more difficult than downsizing an “oversized” engine whose unreasonably high performances are only used in exceptional situations. This leads to a paradoxical situation: most “fuel-efficient technologies” give the highest relative and absolute fuel consumption reduction to large vehicles. It’s on these same large cars that we have the highest leeway to finance the additional cost of fuel-efficient technologies.

What’s more, purchase price sensitivity is much lower for large cars. On this type of car, the additional cost of high energy-efficiency technologies is more easily accepted by the end customer, who will in any case see their investment pay off much more quickly than it would for a small car. Indeed, reducing by 30% the fuel consumption of a vehicle that swallows up 11L of fuel per 100 km saves a lot more money over 100,000 km than does a 10% fuel consumption reduction for a car that consumes 6L/100 km (-3.3L/100 km vs. –0.6L/100 km, or 5.5 times more money saved per km).

When compared with GDI turbo VVL, the fuel consumption reduction provided by VCR remains moderate
for small vehicles. However, these "ultimate" gains are strategically extremely decisive

Even though it is effective, the «roots»
compressor + turbo combination remains expensive for
downsized engines intended for
small popular vehicles

The MCE‑5 integrated compressor project: high
mechanical and thermodynamic efficiency,
ultra-low friction losses when not used (dynamic
seals with no contact when in stand by),
and high-performance innovative valves

The 2-stage turbocharging system selected for
MCE‑5 VCRi protoypes is generally dedicated to
powerful engines (higher than 150kW)

The reduced effectiveness and appeal of fuel-efficient technologies when applied to small cars rather than large cars is true in most cases. For example, a VVL system (Variable Valve Lift) can provide up to 2 times more fuel consumption reduction on a large V6 3.0L than on a small 1.2L engine. Another example: equipping a small low-end vehicle with a Diesel engine provides almost no energy efficiency advantages, while it’s the opposite case for a large vehicle. This is even more off-putting since the cost of Diesel after-treatment systems is becoming increasingly unacceptable for small cars, as is the damping system to manage the high noise and vibration levels of these types of engines. This explains the reversal of trends in Europe: small vehicles are now mostly equipped with gasoline engines.

Since they are more effective and profitable for large cars, it’s easy to understand why new fuel-efficient technologies are firstly commercialized on high-end cars before being applied down the range over the years. This strategy finances these technologies while they are still costly because they are immature, and before being more widely disseminated. It’s possible to produce parts for high-end cars whose cost is not yet optimized and whose quality is “boosted” to avoid any failures. After different industrial development and learning phases, the price of a new technology decreases, which makes it possible to progressively introduce it on less luxurious cars, while preserving an attractive cost/benefit ratio. Unfortunately, these iterations are long and can be an obstacle to technological progress, which in order to have a strong impact on the macro-economic and societal levels, must be applied as quickly as possible to ordinary vehicles that are produced in large numbers.

So, since it’s a new fuel-efficient technology, how long will MCE‑5 VCRi stay too expensive for low-end cars? Probably only for a very short time since it’s inexpensive in absolute terms and very effective in reducing fuel consumption. However, some conditions remain necessary. Let’s take stock of the situation.

VCR’s first objective is hard downsizing and to reach this objective on large vehicles, we use 2-stage superchargers. This configuration can boost the vehicle from the stop position in good conditions, ensure hill-starts, and deliver high torque at low engine speed without sacrificing power at high speeds. Moreover, this type of supercharging system has a shorter response time (turbo lag). Coupled with an intercooler and aftercooler, its isentropic efficiency is higher than 1. Backpressure and exhaust gas temperature are reduced, as is the need to enrich the charge at full power. By combining this supercharging system with gasoline direct injection (GDI), we end up with a downsizing solution that is close to optimal. This type of configuration is unfortunately inappropriate for most small engines.

It’s difficult to plan 2-stage supercharging for small cars, firstly, because the high-pressure turbocharger would be too small with disastrous efficiency. It would have to be replaced by a Roots- or Lysholm-type mechanical volumetric compressor, which are expensive, complex and big given the high mass flows required. These factors make the mechanical compressor + turbocharger pair difficult to accept on very small engines intended for affordable cars. What’s more, if we want to benefit from the advantages of GDI on very small engines, capacities under 260 cc lead to cylinder bores that are too small (under 70mm) which makes it difficult to avoid wall wetting and the associated negative consequences. How can we hard downsize low-power engines in the best functional and economic conditions? By using MCE‑5 VCRi coupled with its MCE‑5 integrated compressor, which is currently being developed.

MCE‑5 VCRi and its integrated compressor should solve almost all the problems associated with the hard downsizing of small engines. On the combustion side, VCR enables the engine to bear extreme loads – up to 35 bar of BMEP – without using GDI, since it’s incompatible with small-bore engines. VCR replaces GDI in this case to manage knock, rumble or hard knock. The accessible BMEP at low speeds increases from 23-24 bar in GDI with a fixed CR of 10:1, to 35 bar in MPFI, thanks to VCR and a minimum CR of 6. On the supercharging side, the problems are solved by the integrated compressor that reduces turbolag to a minimum (roughly 1 second), while being able to guarantee high torques and powers throughout the load/speed range. This mechanical compressor is particularly well adapted to 3-cylinder engines, a 4th cylinder called “compressor” is then added along with a damping plenum (MCE‑5 patent) to overcome the problem of bursts of compressed air of the “4 in 3” type. The resulting engine is an “auto-supercharged” unit that fits into the engine compartment like any naturally-aspirated engine. With this configuration, turbochargers are no longer required, even at high speeds. The exhaust backpressure is low and the maximum exhaust gas temperature is reduced by 300 to 350 degrees, so that charge enrichment can be either reduced or eliminated. The temperatures under the hood are also reduced and the cost of the unit is much lower than that of a 2-stage turbocharger unit or of a mechanical compressor + turbo.

Various preliminary studies have demonstrated the reliability and high energy-efficiency potential of this MCE‑5 integrated mechanical compressor. Several MCE‑5 VCRi engines will be equipped with it in 2011.

In conclusion, whether in the MPFI + integrated compressor version for low to medium range cars or the 2-stage GDI turbo version for medium to high range cars, MCE‑5 VCRi should always lead to the best cost/benefit ratio.

Hard dowsizing applied to small vehicles calls for a volumetric compressor (example: MCE‑5 integrated
compressor) without GDI because of very small bores (could be possible in the near future)