Downsizing

General considerations

Downsizing permits:

	1)	Increasing engines power and torque (to respond to new market demands or to compensate for vehicle weight increase) without increasing cylinder capacity.
	2)	Reducing engines’ capacity at same power.

Whatever the case, downsizing results in an increased engines’ power and/or torque density, which serves one main objective: Fuel Consumption reduction.

Reducing engine capacity at same power permits reducing Fuel Consumption thanks to:

	a)	Pumping losses reduction: Less volume swept on each engine revolution; Higher average load on driving cycle (higher average intake pressure).
	b)	Gases-to-wall heat transfer reduction: Reduced internal surface area; Shorter flame travel distance (faster combustion>> reduced gases-wall heat exchange duration).
	c)	Friction losses reduction: Smaller moving parts.

High torque and density engines also permit to combine downspeeding with downsizing to benefit from an additional friction losses and pumping losses reduction thanks to:

	a)	A reduced number of engine revolutions per kilometer (less total volume swept per kilometer);
	b)	A lower energy consumption per rotation (reduction of moving parts’ speed);
	c)	A higher average load on driving cycle (higher average intake pressure).

This latter point is crucial: downsizing is more efficient when specific power is increased not by engine speed, but by torque increase on the entire engine speed range (by improving cylinder filling and BMEP). Increased BMEP can be obtained by several cylinder filling optimization strategies, among which:

	a)	4 valves per cylinder instead of 2 valves per cylinder;
	b)	Variable valve timing on intake and/or exhaust camshafts;
	c)	Supercharging.

Even if downsizing is often presented as a new strategy, engines have been progressively downsized since the beginning of automotive industry, step by step, depending on technologies availability.

Downsizing benefit is obvious at part loads (example: urban driving cycle). The smaller the engine capacity, the lower its Fuel Consumption at low loads.

Downsizing objective is to give the power of high capacity engines to small capacity engines whithout deteriorating volumetric ratio, because if it is the case, benefit resulting from reduced capacity will not compensate for indicated efficiency reduction due to a lower expansion ratio.

The 4 main limits for downsizing are:

	1)	Knocking;
	2)	Thermal resistance (of motorhead, turbocharger, after treatment devices);
	3)	Mechanical resistance and engine wear (because of increased BMEP);
	4)	Supercharging response time.

The VCR strategy constitutes a major response to points 1, 2 and 4, while the MCE-5 technology provides a major solution to point 3.

Knock limit

FCR engines’ knock sensitivity can be reduced by charge cooling. This can be done by means of air-fuel mixture enrichment and/or Direct Fuel Injection.

Charge enrichment is commonly applied on supercharged engines when running at high power, however, it emits big amounts of HC and CO. Direct Injection constitutes another strategy for cooling the charge thanks to fuel vaporization inside the combustion chamber. However, Direct Injection is costly, and as mixture homogeneity is reduced, particulates generation is widely increased.

Because they emit HC, CO and/or particulates, these strategies will not be suited to future increasingly restrictive emissions standards.

In addition, when applied to FCR engines, these strategies cannot compensate for the strongest limit to knocking: Fixed Compression Ratio. Indeed, the latter has to be set at least at 10:1 to ensure a sufficient indicated efficiency. This remains a strong limit to downsizing.

As an alternative to VCR, Late Intake Valve Closing (LIVC) control has been experimented to reduce the effective compression ratio when supercharging. But this strategy presents a major defect, which highly limits its effectiveness: it combines a technique intended to increase cylinder filling (supercharging) with another one intended to do the contrary (LIVC).

VCR eliminates knock limit

As geometrical volumetric ratio is under control on VCR engines, Compression Ratio no longer determines knock sensitivity, and no longer limits supercharging pressure and specific power: as compression ratio is under control, the engine always operates below knock limit whatever the load.

This allows extreme supercharging by reducing the Compression Ratio (for example down to 8:1) while permitting the engine to operate under a 15:1 to 16:1 Compression Ratio at low loads, providing a significant Fuel Consumption reduction during most of the time under ordinary driving conditions.

The curve shape for Compression Ratio control depending on engine load can be represented as shown on the following graph:

Thermal resistance limit

Thermal resistance counts among strong limits for SI engines downsizing: as power density increases, thermal stress increases for cylinder head, exhaust valves, manifold, pipes and after treatment system.

VCR advantages for reducing high-loaded engine thermal stress

VCR engines allow implementing different strategies to reduce engine thermal stress, that are not available on FCR engines :

	1)	Compression ratio can be increased as engine speed increases (knock sensitivity decreases as engine speed increases): this results in more machine work on the piston and lower exhaust gases temperature at max power;
	2)	It is possible to operate under the best compromise between engine Compression Ratio and ignition advance to reach the best indicated efficiency and then, the lowest exhaust gases temperature.

As a result:

	a)	Higher expansion ratio reduces Fuel Consumption at max power;
	b)	It is possible to increase engine specific power while using the same components and materials with significant cost savings;
	c)	Necessity to increase charge enrichment in order to cool the engine components and the after treatment devices is eliminated or restricted to near-to-max-power operations. This reduces Fuel Consumption (CO2) and pollutants emissions (CO and HC).
	d)	Direct Fuel Injection for charge cooling is no longer necessary to attain high downsizing (avoiding associate costs and particulates generation).

Mechanical resistance limits

A great challenge for highly downsized engines is mechanical resistance: it is necessary to ensure them a lifespan at least comparable to that of present engines. Indeed, in general, supercharged high-loaded engines remain more fragile than naturally aspirated ones.

But the present situation cannot be compared to future situation: current highly turbocharged engines present a similar capacity than that of common engines. In this case, supercharging is used to provide extreme power and torque to respond to exceptional driving conditions (sport cars), but most of the time (during ordinary driving cycle), these engines operate under similar load levels to those of naturally aspirated engines.

In the near future, supercharging SI engines will permit reducing further engine capacity to reduce Fuel Consumption (downsizing). As a result, engines average load will be widely increased. As an example, reducing engine capacity by 50% implies its average load is doubled when operating under the same average engine speed:

As a reminder, on conventional engines the entire torque generated on the crankshaft is due to piston radial stress: cylinder wall is the only surface on which the piston can push on the engine block to generate torque on the crankshaft. Complete explaination in pdf format (Explain_conv.pdf).   This situation is always true whatever the case: torque applied to the crankshaft by inertia forces or by gases pressure. These two forces generate both crankshaft instantaneous torque variations, and torque available for vehicle motion.

As shown on the following graph, gases pressure generates a force between the piston and the cylinder whose curve shape is identical to that of torque on crankshaft:

Downsizing increases average load that ie to say average max cylinder pressure and average BMEP. As a consequence, piston slap is stronger and average piston radial stress is increased as well as first land contact pressure exerted on the cylinder surface.

As a result, cylinder wear and distortion are increased as well as blow-by (loss of torque and efficiency), oil consumption (oil change periodicity, pollutants emissions, reduction of 3-way catalyst effectiveness) and noise.

As a result, the higher the average engine load, the lower the engine lifespan:

The following tables show that the present trend for automotive engines is a constant increase of average load. It can be noticed that present specific power of supercharged Diesel engines is comparable to that of naturally aspirated SI engines, but with a specific torque which is about 1.5 time higher.

Concerning future high-loaded downsized VCR engines, their specific power, specific torque and average load on ordinary driving cycle will be widely increased compared to present SI engines:

Present situation

Future situation

In this context, ensuring durability of future high-loaded downsized engines remains a real challenge, whatever FCR or VCR.

VCR advantages for reducing engine mechanical stress

As it is the case for highly downsized FCR engines, VCR engines will widely increase the engine mechanical stress.

The response to this problem is technological and widely resolved by the MCE-5 technology, which is an effective solution to push back the bounds for engines downsizing.

(see: The MCE-5 technology response to durability of high-loaded engines)

Supercharging response time limits

The higher the downsizing, the lower the engine torque under naturally aspirated operation and the higher its dependence on supercharging to start heavy vehicles from stop to go.

If highly downsized engines’ max torque and power are comparable to those of higher capacity engines, they require short response time turbochargers for transient operation from low to higher speeds. From the technical point of view, it is difficult to design turbochargers for high specific power engines, which present a small turbo lag and provide high torque at low engine speeds.

This is due to the fact that a small turbine would be required to accomodate to low exhaust flow at low speeds, while a big turbine would be necessary to provide the expected max power and torque. If Variable Geometry Turbochargers present the required features, high exhaust temperature of highly supercharged SI engines makes it difficult to guarantee their durability.

In this context, advanced turbocharger technologies have to be combined with complementary strategies:

	1)	It is necessary to design engines that present a high torque at low speeds under naturally aspirated operation. Natural cylinder filling can be improved at low engine speeds by increasing piston speed by means of: A long stroke, A small rod/crank ratio (sinusoidal piston motion highly reduces torque at low speeds). High piston speed has to be combined with an intake system which promotes engine cylinders’ filling at low speeds. All this will permit entering into a virtuous circle: the better the cylinder filling at low speeds, the higher the exhaust flow, the smaller the turbo lag.
	2)	It is possible to increase the average engine speed and exhaust flow when moving the vehicle from stop to go, by means of a low gear ratio for first and second gear (this results in more torque available on the wheel, and a smaller turbo lag).

VCR advantages for reducing the turbo lag

VCR permits reducing the turbo lag effect: Compression Ratio can be reduced to increase exhaust gases enthalpy available for the turbocharger turbine.

VCR also reduces exhaust gases temperature at max power thus making possible to implement more sophisticated turbochargers (example: Variable Geometry Turbochargers).