Advancements in Automobile Technology

Advancements in Automobile Technology

VARIABLE COMPRESSION RATIO HIGH CHARGE COMBUSTION IGNITION ENGINE 

CONTENTS

Introduction
Abstract
Demand for variable compression ratio engine
Methods of VCR control
Engine construction details
Effects of VCR
Performance of VCR
Emissions
Advantages of VCR over FCR
Conclusion
Bibliography

INTRODUCTION:

Ever increasing requirements to provide for manufacturing of economy and ecology friendly vehicles have resulted in using the most complicated and intellectual means of internal combustion engine development. Nowadays new ideas, which couldn’t be discussed 10-20 years ago, are considered by automotive manufacturers. Conventional gasoline engine encounters many drawbacks such as low power, high fuel consumption at different load conditions. Many researches has found a solution for this problem which may give performance similar to that of diesel engine with direct injection which should also satisfy the recent emission norms.

ABSTRACT:

Conventional gasoline engines operate at a fixed compression ratio, which is set low enough to prevent premature ignition of the fuel, or “knock,” at high power levels under fast acceleration, high speeds, or heavy loads. Most of the time gasoline engines operate at relatively low power levels under slow acceleration, lower speed, or light loads. If the compression ratio were increased at low-power operation, gasoline engines could achieve higher fuel efficiency. A variable compression ratio (VCR) engine is able to operate at different compression ratios, depending on the particular vehicle performance needs. The VCR engine is optimized for the full range of driving conditions, such as acceleration, speed, and load. At low power levels, the VCR engine operates at high compression to capture fuel efficiency benefits, while at high power levels, it operates at low compression levels to prevent knock. To further improve fuel economy, the VCR engine is small, with about one third the displacement volume of a conventional gasoline engine. A supercharger boosts engine peak power when needed for occasional hard acceleration or hill climbing. Downsizing results in better efficiency. With small size of the engine fuel efficiency is good at low and part loads and at high loads it uses supercharger for increasing power, thus obtaining good power and fuel efficiency from a small engine. Many methods have been devised for varying the compression ratio. They will change the compression ratio continuously depending on the load and speed conditions. Thus a singe downsized engine contributes for good power output when needed and better fuel economy with reduced emissions.

DEMAND FOR VARIABLE COMPRESSION RATIO:

Fixed compression ratio is always a constraint for supercharging or turbocharging engines. To prevent excessive pressure in combustion chamber, hence pre-ignite ("knocking") and overheat to cylinder head, turbo/supercharger engines always employ a much lower compression ratio than normally aspirated engines so that the total pressure won’t exceed the limit when the boost pressure is added. The problem is, when the charger (especially is turbocharger) is not yet getting into full boost, that is, at low and mid rev, the combustion runs at lower compression ratio than normally aspirated engines. Therefore power efficiency at low speed is even lower than normally aspirated engines. To avoid this problem the compression ratio has to be reduced so that knocking will not occur. At the same time the performance is not good as the compression ratio is fixed which is not favorable for certain load conditions. To achieve better power output and fuel efficiency the variable compression ratio has to be employed.

METHODS OF VCR CONTROL:

Variable compression ratio (VCR) technology has long been recognized as a method for improving the fuel economy of SI engines. In order to vary the compression ratio, some method of varying the geometric compression ratio through changing the clearance volume is required. There are several ways of doing this; including modification of the compression ratio by moving the cylinder head, variation of combustion chamber volume using a secondary piston or valve, variation of piston deck height, modification of connecting rod geometry, moving the crankpin within the crankshaft, and moving the crankshaft axis. The potential of these technologies needs to be evaluated by a trade-off between cost and consumption benefit. Out of these different methods of controlling compression ratio only few are practically feasible and proved to be profitable. The main principles of compression ratio control are by rod-crank mechanism, multilink rod-crank mechanism, gear based mechanism, use of eccentric bearings, articulated engine parts, use of additional pistons etc., the schematic explanation of these principles are shown below.

A	Articulated cylinder head	D	Multilink rod-crank mechanisms
B	Hydraulic pistons	E	Additional piston in cylinder head
C	Eccentrics on bearings	F	Gear-based mechanisms

ENGINE DETAILS:

ARTICULATED CYLINDER HEAD TYPE:

Top: high compression ratio; Bottom: low compression ratio

GEAR BASED MECHANISM:

It consists of an all-in-one engine block integrating both innovative components that transmit power from the piston to the crankshaft, and exclusive actuators, which permit controlling the engine Compression Ratio. Main components of the engine block are shown in the following scheme:

The engine block is a combination of two principles, which constitute the technical basis for the entire automotive industry: gears and rod-crank mechanism:

This arrangement presents exclusive features and advantages:

	1)	The engine remains compatible with all combustion chamber shapes and cylinder head designs;
	2)	Its piston kinematics remains exactly the same than that of a conventional engine with the same rod/crank ratio. This remains true whatever the Compression Ratio
	3)	The integrated exclusive hydraulic actuators provide a wide range, precise and continuous Compression Ratio control for each cylinder of the engine.
	4)	The crankshaft is particularly rigid
	5)	The crank-case rigidity is at least equivalent to that of conventional engines, providing a rigid and precise bearing line and an optimum geometrical environment for all moving parts.
	6)	The piston is roller-guided and is no longer subjected to rod thrust (no piston radial stress) or to piston slap. Forces that generate torque on crankshaft are entirely assumed by rollers: this arrangement reduces friction losses and widely extends the cylinder lifespan. This constitutes a strong response to the durability problem of highly downsized high-loaded engines;
	7)	All components are enclosed in the engine block and no moving part is visible from the outside. The engine is fixed into vehicles and connected to gearbox, pipes and peripherals as if it were a conventional engine

The engine block responds to all requested features for Compression Ratio control of future VCR engines:

	1)	A wide range and continuous Compression Ratio control from 7:1 to 20:1;
	2)	A high reactivity CR control (100ms and lower to move from CR max to CR min);
	3)	A cylinder per cylinder Compression Ratio adjustment;
	4)	A high precision control (about 0.01 mm);
	5)	A low energy consuming actuation.

EFFECTS OF VCR:

Although a variable compression ratio is what makes the VCR engine unique, the fuel efficiency of a conventional naturally aspirated engine would only improve 4 - 5 percent if it were equipped with a variable compression system. The full potential of variable compression can only be realized when it's used in combination with reduced engine displacement and high supercharging pressure.

1. Reducing the engine displacement

A conventional four-stroke gasoline engine is most efficient (maximizing the energy in the fuel) when it is running at a high load. A small engine must work harder and run closer to full load if it is to perform the same work as a bigger engine, which utilizes only part of its maximum capacity. The small engine often extracts more energy from every drop of fuel.

One reason for this is because the pumping losses are lower in a small engine. Pumping losses arise when the engine is running at low load and when its fuel consumption is relatively low. In order to maintain the ideal air-to-fuel ratio (14.7:1), the air supply must be restricted by reducing the opening of the butterfly valve in the air intake.

However, this means that the piston in the cylinder is under a slight vacuum during the suction stroke, when it is drawing air into the cylinder. The extra energy needed for pulling the piston down is known as the pumping loss. Since a small engine frequently runs at full load and the throttle is therefore more often fully open, the pumping losses in the small engine are usually lower than they are in a big engine.

Additionally, a small engine is lighter, has lighter internal reciprocating mass and has lower frictional losses. Therefore, a small engine is generally more efficient than a big engine.

2. Supercharging

Although a small engine is efficient, it is not powerful enough to be used for anything other than powering small, lightweight cars. By supercharging the intake air and forcing more air into the engine, more fuel can be injected and burned efficiently. The engine then delivers more power for every piston stroke, which results in higher torque and horsepower output. By supercharging the engine only at greater throttle openings when extra power is really needed, the fuel economy of a small engine can be combined with the greater performance of a big engine.

3. Variable compression - pearl of wisdom

The compression ratio is one of the most important factors that determine how efficiently the engine can utilize the energy in the fuel. The energy in the fuel will be better utilized if the compression ratio is as high as possible. But if the compression ratio is too high, the fuel will pre-ignite, causing "knocking," which could damage the engine. In a conventional engine, the maximum compression ratio that the engine can withstand is therefore set by the conditions in the cylinder at high load, when the fuel and air consumptions are at maximum levels. The compression ratio remains the same when the engine is running at low load, such as when the car is traveling on the highway at constant speed.

Due to its variable compression ratio, the VCR engine can be run at the optimum compression ratio of 14:1 at low load in order to maximize the use of the energy in the fuel, and the compression ratio can then be lowered to 8:1 at high load to enable the engine performance to be enhanced by supercharging without inducing "knocking."

EXPERIMENTAL RESULTS:

The VCR concept enables the fuel consumption of a conventional naturally aspirated engine to be reduced by up to 30 percent without impairing the engine performance. The five-cylinder SVC engine developed by Saab has a displacement of 1.6 litres and is as fuel-efficient under normal conditions as a conventional 1.6-litre engine, but can deliver the power of a 3-litre engine whenever the need arises. The emissions of carbon dioxide (CO2) are reduced proportionately to the fuel consumption, while the CO, HC and NOx emissions will enable the SVC engine to meet all current and proposed future legal requirements.
The unique feature of the SVC engine and the one which is the key to high efficiency is that the engine has a compression ratio which is variable. The fixed compression ratio of a conventional engine is a compromise between the needs in a wide variety of operating conditions - in stop-go city traffic, in highway motoring at constant speed, or in high-speed motorway journeys. As opposed to this, the compression ratio of the SVC engine is continually adjusted to the optimum value for the prevailing conditions.

Engine displacement                                 1.598 liter

Number of cylinders                                  5

Cylinder bore                                             68 mm

Piston stroke                                              88 mm

Compression ratio                                      8:1 to 14:1, depending on engine load

Max. Compressor boost pressure                2.8 bar (40 psi)

Max. Monohead tilt angle                           4 degrees

Maximum torque                                        224 Ib.-ft.

Maximum horsepower                                225 hp

PERFORMANCE OF VCR:

Fuel Consumption reduction provided by advanced FCR engines or VCR engines mainly depends on max power, vehicle characteristics, and driving cycle. Main tendencies for VCR potential fuel saving compared to present naturally aspirated engines can be presented as shown on the following graph: Fuel Consumption reduction potential depends on max power on one hand, and on strategies combined with VCR on the other hand:

Fuel Consumption reduction provided by VCR not only depends on engine max power but on engine power and torque oversizing. In other words, the further the engine power and torque from vehicle’s needs under ordinary driving conditions, the higher the Fuel Consumption reduction provided by downsizing.

As knocking is under control on VCR engines, high supercharging is possible with no need for a low fixed Compression Ratio. As a result, downsizing ratios of about 50% are possible for high powered vehicles. In addition, VCR provides an additional efficiency gain thanks to high expansion ratios at part loads that compensate for pumping losses. High expansion ratios at part loads will permit to benefit from a better efficiency at low loads than at high loads.

Low loads are highly represented on common driving cycle; high expansion ratios at part loads have a significant impact on Fuel Consumption reduction.

EMISSIONS:

For automotive engines, the great challenge is not only to confirm to future pollutant emissions standards: this is already possible under good conditions. The great challenge is to conform to pollutants emissions standards while responding to CO2 emissions reduction objectives.

Indeed, there is a contradiction between reduction of pollutants emissions, in particular NOx, and reduction of CO2 emissions. This for two reasons:

	a)	Increasing combustion pressure permits improving indicated efficiency, but resulting higher combustion peak-temperature generates a larger amount of NOx;
	b)	If combustion under excess air permits reducing pumping losses to improve the efficiency of vehicles used under continuously variable loads, 3-way catalysts cannot reduce NOx in oxygenated exhaust gases

Highly downsized VCR engines present unique features: whatever Otto or Otto-Atkinson, they provide a high fuel consumption and CO2 reduction while operating under stoichiometric combustion with no need for Direct Injection.

This presents two main advantages:

		3-way catalyst is fully operational to reduce emissions of regulated pollutants (no need for NOx traps and associate defects: extra-costs, sulfur fuel sensitivity and energy consumption for regeneration);
		Avoiding Fuel Direct Injection implementation avoids associate extra costs and particulates emissions.

Concerning this first point, VCR provides both low pollutants emissions levels (CO, HC and NOx),AND low CO2 emissions levels.

But VCR goes further and provides a better control over pollutants generation and after treatment than conventional FCR engines. Indeed, VCR allows controlling main parameters that determine pollutants generation and after treatment devices operation, characteristics and durability.

These parameters are the following:

		Combustion pressure and temperature;
		Expansion ratio and indicated efficiency;
		Combustion chamber volume.

Many relations exist between these 3 main parameters, which allow for several strategies to control pollutants generation and their after treatment

ADVANTAGES OF VCR OVER FCR:

Ü Increased effective expansion ratio increases the indicative efficiency

Ü Decrease in the cylinder volume (downsizing) results in

· Reduction in pumping losses

· Reduction in flame travel distance- decrease in the tendency of Knock

Ü Downsizing results in better fuel consumption-30% more than conventional engines

Ü Decrease in CR at high loads reduces peak pressure- decrease in Knocking

Ü Increased power due to high supercharging pressure

ü As 3-way catalyst is fully operational, VCR provides low emission levels of HC, CO, NOx and CO_2.

CONCLUSION:

The Variable Compression Ratio technology opens the way to the implementation of high power fuel-efficient engines on a wide range of vehicles.