variable compression ratio engines

04/10/2023 Variable compression ratio 1

Variable Compression Ratio Engines

Md.Azheruddin08MES37 ,B.tech


Abstract• Generally, the operating conditions of SI engines vary widely, such as stop and go city tra c, ffi

highway motoring at constant speed, or high-speed freeway driving. In a conventional SI engine, the maximum compression ratio is set by the conditions in the cylinder at high load, when the fuel and air consumption are at maximum levels. If the compression ratio is higher than the designed limit, the fuel will pre-ignite causing knocking, which could damage the engine.

• Unfortunately, most of the time SI engines in city driving conditions operate at relatively low power levels under slow accelerations, low speeds, or light loads, which lead to low thermal e ciency and hence higher fuel consumption. As the engine load decreases, the temperature in ffithe end gas drops, so that high compression ratio could be employed without the risk of knocking in naturally aspirated or boosted engines.

To further improve fuel economy, the VCR engine is small, with about one-third the displacement volume of a conventional gasoline engine.

Variable compression ratio engines are those engines that have varied compression ratios at different engine speeds .used mostly in the diesel engine, these engines have high values of economy compared to the ordinary engines. Compression ratio of an engine is the relative amount to which the volume available is compressed

This compression ratio that is available is to be high when the engine is working at low speeds. When the speed is increased, the efficiency value is decreased .the requirement is a low compression ratio and that too by increasing the clearance volume.

Since the engine is running at low speeds at the first, the time availability is high, but when ever the speed increases the available time for combustion is less and hence the volume required is high.


Introduction To VCR Concept The first being built and tested by Harry Ricardo in the 1920s. This work

led to him devising the “Octane Rating “ system that is still in use today.

The concept of variable compression ratio (VCR) promises improved engine performance, e ciency,and reduced emissions. As a result, at low loads, the greater the compression ratio, ffithe shorter is the combustion time. Time loss is subsequently reduced. Therefore, it seems reasonable that fuel consumption rate is lower with high compression ratios at part load.

Variable Compress

ion

High Charging

DownSizing


Introduction To VCR Concept(contd) The main feature of the VCR engine is to operate at di erent compression ratios, ff

depending on the vehicle Performance needs. At low power levels, the VCR engine operates at a higher compression ratio to capture high fuel e ciency benefits, while at ffihigh power levels the engine operates at low compression ratio to prevent knock.

The optimum compression ratio is determined as a function of one or more vehicle operating parameters such as inlet air temperature, engine coolant temperature, exhaust

gas temperature, engine knock, fuel type, octane rating of fuel, etc. It has been proven that a VCR engine develops much more power for the same engine

dimensions. SAAB has recently been involved in working with the 'Office of Advanced Automotive

Technologies', to produce a modern Petrol VCR engine that showed an efficiency comparable with that of a Diesel


Octane Rating Octane Does Matter:

Octane rating reflects the quality, purity, refinement, efficiency and heat bearing capacity of Fuels.

What is an octane rating (or number) ?

The simplest definition is that such rating/number is a measure of a fuel’s resistance to detonation during combustion.....or the fuel’s anti-knock quality.

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance."


Octane Rating Calculation Methods RON: (Research Octane Number). Most common fuel used in Europe, Australia and

some other Countries. It is determined with measurements of fuel behavior in a variable compression engine. Results are compared with other iso-octane fuels, wish means, compared with other fuel types with equal number of similar octane molecules present, but not the of the same organic compound.

MON: (Motor Octane Number). Determined also with a variable compression engine, but with the fuel already preheated, variable ignition timing and higher RPMs than the RON method. This method is more precise on determining fuel behavior on an loaded engine.

PON or AKI: (Pump Octane Rating or Anti-Knock Index): This is the method used in USA and Canada, expressed as [R+M]/2, wish means RON number plus MON number, divided by 2. In other words, it is the average between both methods above.

Equivalency: So, due to the fact that RON is always from 4 to 5 points higher of its equivalent to PON

or AKI, the number conversion between USA and Europe octane ratings will approximately be as follows:

USA (PON) -> Europe (RON) 87 -> 91

89 -> 93 91 -> 95 93 -> 98


Octane Rating For IndiaIndia:

India's Ordinary And Premium Petrols are of 89–91 RON. The premium petrols are generally ordinary fuels with additives, that do not really change the octane value. Two variants, "Speed 93" and "Speed 97", were launched, with RON values of 93 and 97. India's economy-class vehicles usually have compression ratios under 10:1, thus enabling them to use lower-quality petrol without engine knocking


Octane Rating Chart According To Compression Ratio’s


Main objectives for the VCR system The VCR system is not allowed to add any kind of friction to the

engine under operation that increases parasitic losses.

The VCR system must not affect the combustion chamber in a negative way, detoriating engine efficiency.

The VCR system should be infinitely variable during operation between CR 8:1 and 14:1


Why Atkinson Cycle In VCR Implementation ? In the Otto cycle, if the expansion is allowed to completion at point 4 and

heat rejection occurs at constant pressure, the cycle is called the Atkinson cycle.

The original Atkinson cycle piston engine allowed the intake, compression, power, and exhaust strokes of the four-stroke cycle to occur in a single turn of the crankshaft and was designed to avoid infringing certain patents covering otto cycle engines.Because of the unique crank shaft design of the Atkinson, its expansion ratio can differ from its Compression ratio and, with a power stroke longer than its compression stroke, the engine can achieve greater thermal efficiency than a traditional piston engine.


Piston-Crank Mechanism By applying multi-link mechanism, significant improvement of engine vibration coupled with

friction reduction (basic condition of engine mechanism) have been obtained. And in addition, function of variable compression ratio control under engine running condition has been achieved.


Low vibration nature of multilink mechanism Piston motion in the conventional mechanism is essentially asymmetrical, with rapid motion

occurring at top dead center. That unbalance deteriorates engine vibration in the case of conventional 4-cylinder engines.

With the multilink VCR mechanism, the piston’s reciprocal action achieves ideal simple harmonic motion, resolving the unbalance. This enable low vibration property of in-line 4 cylinder engine close to that of V type 6 cylinder engines.


Features of VCR piston-crank system The newly developed piston/crank mechanism (multi-link type) enables optimum compression

ratio control. If We Assume ,the fuel economy of a non-turbo engine and the power output of a turbocharged engine can be achieved in one engine. By the ideal piston motion of multi - link, significant vibration reduction has been obtained.


Need For VCR Technology The present challenge in automotive engine technology is the improvement of thermal

e ciency and hence the fuel economy and lower emission levels. One of the key features ffia ecting thermal e ciency is the compression ratio, which is always a compromise in fixed ff fficompression ratio spark ignition (SI) engines.


Compression Ratio

It is the ratio between the volume of the cylinder , when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke.

The compression ratio is a single number that can be used to predict the performance of any engine (such as an Internal-Combustion Engines or a stirling Engine).

It is given by the expression

Compression Ratio = (Volume Before Compression) / (volume After Compression) Compression ratio and overall pressure ratio are interrelated as follows:

Compression Ratio

1:1 3:1 5:1 10:1 15:1 20:1 25:1 35:1

Pressure Ratio

1:1 2:1 10:1 22:1 40:1 56:1 75:1 110:1


Variable Compression Approach There are a number of different mechanical approaches that are

being explored to vary engine CR. Among them the following is more efficient:

Moving Head : The moving head concept (Saab Automobile AB) combines a

cylinder head with cylinder liners into a monohead construction, which pivots with respect

to the remainder of the engine. The lower half of the block includes the crankcase and engine mounts, and carries the crankshaft, gear box, oil cooler, and auxiliaries. The upper half includes the cylinders, their


Variable Compression Approach (Contd) liners, camshafts, and an integrally cast cylinder head.This part is referred to as the

monohead .Saab has enabled a tilting motion to adjust the e ective height of the piston crown ffat TDC. The linkage serves to tilt the monohead relative to the crankcase in order to vary the TDC position of the piston. By means of actuator and linkage mechanism the compression ratio can be varied from 8 to 14. A screw type supercharger provides a 2 : 1 boost pressure when wideopen throttle conditions occur . This system gives wide fuel flexibility, with reduced CO emissions proportional to fuel consumption. Saab recognized that the fuel efficiency of the VCR engine would be low without high-pressure supercharging


Advantages An overall fuel consumption reduction up to 27 %. Pollutant emissions like co2 is been Reduced. This new technology must be able to fulfill future emission standards and improved drivability

of the vehicles. Due to use of higher compression ratio at low loads the VCR engine has a good cold starting

and idling performance at low ambient temperatures. As a compression ratio is reduced the first stage of combustion increases while the second

state of combustion decreases but the overall duration is release is shortened, i.e. as the load is increased the ratio heat released also decreased. This results in smoother combustion at both lower and higher compression ratio.

Its the fact that the expansion ratio is larger than the compression ratio in the atkinson cycle that makes it more fuel efficient.


DisAdvantages The disadvantage of the variable

compression ratio is that it might be too expensive for some economy cars.


Prestigious Car Manufacturers Honda : At the SAE World Congress Honda let loose a paper on their

development of a variable compression ratio (VCR) engine. Higher compression ratios are key in increasing the fuel

efficiency of internal combustion engines because, as a general rule, the higher the compression ratio the better the thermal efficiency of an engine is.

Nissan , Toyota , Ford


Conclusion

In addition to improve Fuel Consumption of the engine to get overall increase in the efficiency of the Engine at the required Intervals of Loads.VCR intentionally make the Reduction Of Noise/ Vibration Of the Engine.In Future The Optimised Usage Of this New Technology all allover the World will fulfill future Emission Standards and Improved drivability .


Bibilography…. John B Heywood, Internal Combustion Engine Fundamentals, 1988, ISBN 0-

07-100499-8 J R Clarke, R J Tabaczynski, US Patent 6135086, 2000-10-24, Internal

combustion engine with adjustable compression ratio and knock control. J Beroff, European Patent 1143127, 2001-10-10,Internal combustion engine

with variable capacity and compression ratio. G Rapan, I Rapan, Romanian Patent RO115662B, 2000-04-28, Variable

compression ratio engine. Moteki, K., Aoyama, S., Ushijima, K., Hiyoshi, R., Takemura, S., Fujimoto, H.,

and Arai, T. A study of a variable compression ratio system with a multi-link mechanism. SAE paper 2003-01-0921, 2003.

Haraldsson, G., Tunestal, P., Johansson, B., and Hyvonen, J. HCCI combustion phasing in a multi-cylinder engine using variable compression ratio. SAE paper 2002-01-2858, 2002.

Clarke, J. R. and Tabaczynski, R. J. Internal combustion engine with adjustable compression ratio and knock control. US Pat. 6135086 (24 October 2000).

Sykes, R. G. and Tickford. Methods to reduce the fuel consumption of gasoline engines. In Proceedings of Conference Engines Expo 2000.

http://virtualpowertrain.com/papers/Investigation%20and%20Development%20of%20an%20Atkinson%20Cycle%20Engine%20with%20the%20Aid%20of%20Artificial%20Neural%20Network.pdf

http://www.nissan-global.com/


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