Slide 1

VEHICLE POWER PLANTSCh-7-W-13-14

VCR and Hyper bar engines

High Pressure Turbocharging 112Construction and operation of VCR enginesConfigurations used to obtain variable compression ratios.BICERA pistons mechanism and operationsAdvantages and draw backs of VCR engines with BICERA pistonsNumerical problems to analyze the concept

Hyper bar engines principle of operation and necessityConfigurationCalculationsAdvantages and drawbacks

Problems of Turbocharging a Naturally Aspirated (NA) engineHigher pressure and temperature

a. Cylinder head gaskets tend to leak

b.Big/more cylinder head bolts required

c.Piston ring pack likely to run hotter; piston ring friction also increases due to higher radial pressure on rings-requiring cooling by oil jets.

d.Hotter cylinder head temperature controlled by:-After charge cooler-10 cooling results in 20 cooling of compressed air.Cavities around combustion chamber enlarged for better coolingLarge water pump and radiator required.

2.For burning more fuel, FIP and Injectors need to be re-designed or, adjusted3Solution to Problems of Turbocharging an NA engine

AFVs are highly boosted to achieve maximum power output; running at higher temperatures and pressure.

To overcome the above problems, following steps can be taken:-

Reduce Compression ratio (CR)- It reduces the peak pressure and temperature but leads to lower th.

Boost now at lower P & T to get maximum power o/p.

4PRESSURE CHARGING: LOWERING THE CRIn a hot loaded engine, lowering the CR much below normal will ensure compression ignition.

But low CR will introduce the problem at cold start and, perhaps at sustained idle.

To cater for the above, artificial charge warming may be necessary; by at least four methods:- Manifold heating.Exhaust recirculation. VCR Engine.Hyper bar system. MANIFOLD HEATING

Fuel sprayed into the incoming air charge and igniting it electrically if engine temperatures are low enough to make compression ignition uncertain. At high load conditions, this extra fueling is automatically turned off.EXHAUST RECIRCULATIONAllows some of the exhaust gases to re-circulate into the engine intake.

Done by advancing the valve-timing of the engine during cranking. As a result the not-yet-firing engine compresses its air charge on the compression stroke more than it expands it during expansion.; so it pumps back hot air back into its own inlet.

This can achieve light load running indefinitely, but achieving of cold start is less obvious.

Higher Power Output

For higher power O/P, we need higher mep.

MEP upto 30 bar is achieved in some engines without increasing the peak pressure (110-140 bar).

MEP depends upon the amount of fuel burnt. This in turn means:-

More amount of airMore mass of airHigher density of airHigher volume of air at the end of compression per cycle.

Therefore, for burning maximum fuel for maximum power, we need:-Pc to be highest-not desirableTc to be lowestVc to be highest possible (clearance volume to be maximum)-Low CR

8VARIABLE CR PISTONCR continuously adjusted while the engine is running- high for start and low for heavy load.

Accomplished by special piston- VCR

CR maintained between 8:1 to 24:1.

BICERA piston

Used on AVCR-1360 ENGINE Max pressure inside the combustion Chamber 140 bar

VCR Engines-BICERA Piston

Principle of Operation.

It runs at a CR of 8:1 (about half of the common diesel engines); so clearance volume is maximum (double) and,

Temperature is just sufficient to sustain the combustion.

Thus, 2 requirements out of the 3 are met with low Pc accepted.The problem of cold starting and the idle/part load is taken care of by having high CR which goes on decreasing as the load picks up to the maximum.

Maximum pressure never exceeds 140 bar.10

Construction.

C- Upper oil chamberD- Lower oil chamberH,J- one way inlet valvesL- Discharge valve; adjustableK- Small orifice to drain oil if pressure in D exceedsG- Oil passage way

Two piston rings between shell and the carrier to prevent oil leakage.

H G C

F E L

J

D K

E

A slot in the inner piston, keyed from the outer piston to avoid rotation but, outer piston can move up and down.

Outer piston- malleable cast ironInner piston- forged aluminium11

H G C

F E L

J

D K

E

Operation.

During Cranking.

As the carrier connected with the con. rod comes down, shell tends to lag behind. Hence pressure in lower oil chamber D increases and oil leaks out to the sump. Also pressure in the upper oil chamber C decreases so it is filled up with more oil.

So the gap between the outer and inner piston increases- Vc is minimum and CR maximum.As Carrier moves up, shell tries to lag behind but does not: due to oil pressure in C.

But since the motoring air pressure at the top of the piston is less than the oil pressure in C, both shell and carrier move together and the CR is maintained at 22:112

H G C

F E L

J

D K

E

Operation.

2. During Loaded Condition.

As engine fires and picks up, the combustion pressure in the combustion chamber above the piston increases upto 140 bar.

Now this pressure exceeds the spring force of valve L. Thus oil from C drains out to the sump. Thus decreasing the gap between the outer and inner pistons.

Now the clearance volume VC increases thereby reducing the C.R13Advantages of VCR Engine.

Better overall efficiency. At full load, th is less due to reduced C.R but power O/P is more. But at idling and part loads th is better- we know AFVs run quite a lot on idling/part load.

Increases BMEP. Rate of increase of bmep is more; compensating for the low th at full load by giving out more power O/P.

Lower sfc. Less than 0.20 kg/km.hr.

High Power at full load. Compensates for lower th.

H G C

F E L

J

D K

E

5.Increased Power O/P. Upto 50% additional due to higher maximum peak pressure. It also depends on of the turbocharger and inclusion of a charge cooler.14Advantages of VCR Engine

6.Lower Bulk. Due to more power O/P.

7.Better Acceleration. Due to increased boost in bmep.

8.Better Cooling of Piston Ring Pack. By circulation of oil through G.

9.Better Cold Starting. Due to high C.R. With simple glow plug, engine can be started at temperatures as low as -400 to -500 C

H G C

F E L

J

D K

E

15Disadvantages of VCR Engine

Heavier Piston. 80-100% more weight leading to 50-75% more mass of reciprocating parts. Thus we need sturdier parts.

Cost of piston. Very high.

High Capacity Lub. Oil Pump Required. They consume more power and space.

Complicated Design.

Reliability. Questionable.1617HYPERBAR TurbochargingThis system has been developed to overcome some of the practical difficulties inherent in using conventional turbocharging system on very high output engines, particularly those required to operate over a large speed range.These difficulties may be summarized as under:-Low turbocharging efficiency at very high pressure ratios.High peak cylinder pressure.Low boost at low engine speed.Poor starting and light load running with low engine compression ratio (CR).Insufficient compressor map width at high pressure ratio.Slow turbocharger acceleration.

18HYPERBAR Turbocharging..This system uses an additional combustion chamber (CC) placed between the exhaust ports of the engine and the turbocharger turbine. Additional fuel is burnt in this CC to increase turbine and therefore, compressor work, overcoming disadvantages 1 and 3.In order to provide sufficient air for the auxiliary CC, some air from the turbocharger compressor by-passes the engine directly to this CC. This bypass is also used to restrict compressor mass flow variation as engine speed and engine air flow change, overcoming disadvantages number 5.Ch-7-W-13-14-VCR and Hyperbar engines Prof (Col) GC Mishra19

HYPERBAR Turbocharging.Figure shows engine and compressor air flows, superimposed on a very high pressure ratio, single stage compressor map. This means that the compressor is easily matched to the engine and that the compressor may be designed for high efficiency and pressure ratio without sacrifice to achieve a wide flow range.Ch-7-W-13-14-VCR and Hyperbar engines Prof (Col) GC Mishra20

HYPERBAR Turbocharging..The hyperbar turbocharging system is designed to operate with very high compressor pressure ratio and very low engine compression ratio, overcoming disadvantage number-2For example, the Molchior and Talamon report tests in which the Hyperbar system is used to raise compressor pressure ratio from 2.8 to 7.26 (using a 2-stage system), while engine CR is reduced from 13:1 to 7:1. BMEP is increased from 17 to 30 bar with no increase in maximum cylinder pressure (140 bar).21HYPERBAR Turbocharging...An additional disadvantage of conventional system listed above (4) is poor starting and light load running with low engine CR. The Hyperbar uses an electric starter motor to rotate the turbocharger rotor, with the engine air by-pass open.

22HYPERBAR Turbocharging....The air flow is sufficient to enable to the auxiliary CC to be started. The turbocharger starter motor is then decoupled, allowing the turbocharger to operate like a free-running gas turbine.The engine may then be cranked by normal means, and air is drawn into the CCs. Normally this air would be at atmospheric pressure and temperature and the CR of the engine would not be high enough for self-ignition. However, since the gas turbine part is self-sustaining, a reasonable boost temperature and pressure can be developed before the engine fires. By by-passing the charge air-cooler, warm air from compressor exit is delivered to the cylinders aiding in in-cylinder ignition. The charge air cooler may also be by-passed during light load operation to maintain high air temperature and good combustion. At higher loads, a gradual transition to full air-cooling is required to limit the thermal loading of the engine.

23HYPERBAR Turbocharging.....The auxiliary CC can also be used to improve turbocharger and engine response to rapid load and speed demand changes. This can be achieved by increasing the auxiliary CC fueling rapidly to accelerate the turbocharger when required., and by maintaining fuelling at light load and low speed to prevent the turbocharger speed from ever falling to a low value. This overcomes the final disadvantage number-6.Principle of Operation

The Hyperbar system is based on the use of a high pressure turbocharger driven not only by the exhaust gases but also by the additional energy supplied by a gas turbine type combustion chamber operating in parallel with the engine.

CR can be as low as 7:1 and boost pressure more than 5 bar. This system uses lean air fuel ratio and this gives greater specific power output without increase in mechanical or thermal loading

The turbocharger specially developed for the UD V8 X 1500 engine by Turbomeca provides a compression ratio of 7.9:1 and in its second TM 307 B version has a single-stage centrifugal compressor driven by a two-stage axial-flow turbine.24

Operation.

1.At the starting of the engine, compressor is turned on electrically. All the air is given to the auxiliary comb. Chamber where fuel is burnt. All the exhaust gases are fed into the turbine till the O/P pressure of the compressor reaches 5 bar.Now when the engine is cranked, high pressure air is available in the main comb. Chamber; temperature is sufficient for combustion; fuel injected to start the engine.

Very low C.R (7:1) provides maximum VC, with high boost pressure PC with TC minimum (along with charge cooler)

25Advantages of Hyper bar Engines. Excellent transient response- turbo-lag phenomenon is eliminated. Response is good at all loads due to the auxiliary comb. Chamber; surge free operation.High bmep from 25 to 30 bars with peak pressure limited to 140 bars.High power to weight ratio.Moderate thermal loading.No auxiliary engine required.Disadvantages

High specific fuel consumption and low thermal efficiency- (Running engine at low C.R constantly).

Complex design and poor reliability (prone to malfunctioning).

Use. This system is being used in AMX-40 and LECLERC tanks.

2627THANK YOU The End

QUESTIONS ? ?CONCLUSION