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Q1.

a) What is Carburetion?
b) Describe the basic functioning of a carburetor with the help of neat sketch?
c) Describe different elements of an advanced carburetor. Describe its functioning under various condition of load changes?
d) Write down advantages and disadvantages of EFI?
e) Enlist different components of electronically controlled carburetor and describe their function?

Q2.
a) Describe different water cooled system used for IC Engine cooling?
b) What are the advantages of field Installed Radiators over spray pond cooling?
c) A 10 MW Diesel Generator is operating with 35%, 98% and 90% Thermal efficiency, Mechanical efficiency and electrical efficiency. Assuming 75% of thermal losses are carried away by the cooling water. Calculate the capacity of rise in temperature in 20 degree Centigrade and specific heat of water is 4.2 KJ/kg^oK.

Q3.
a) What is different methods of lubrication used in IC Engines?
b) Describe wet sump lubrication system with the help of a neat sketch?
c) Describe the advantages of a Dry Sump system of Lubrication over wet sump system. Draw a neat sketch of a dry sump system.

Q4.
a) Desribe valve timing diagram of a 4-stroke S.I. Engine?
b) Give the reason, why Inlet valve starts opening by the start of suction stroke in the cylinder of over lapy with opening of suction and discharge valves.

Q5.
a) Enlist the main causes of environmental pollution. Describe the role of I.C. Engine power plants in spread of pollution.
b) Describe the methods of pollutants detection. What are the types of pollution detection equipments.
c) What is a Catalytic converter? On what principle does it function? What are causes for loss of its effectiveness?
d) What is exhaust gas recycle EGR. How does it effect pollution control in I.C. Engine?

Q1a)
In engines using volatile liquid fuel, the mixture of fuel and air is formed outside the engine cylinder. Mixture formations is not usually complete outside the cylinder and the fuel droplets which remain in suspension evaporate and mix with air during admission and compression processes. The process of formation of combustible fuel air mixture by mixing proper amount of fuel with air before admission to engine cylinder is called carburetion.
 

Q1.b)
It is a device for supplying a spark-ignition engine with a mixture of fuel and air. Components of carburetors for automobile engines usually include a storage chamber for liquid fuel, a choke, an idling (or slow-running) jet, a main jet, a venturi-shaped air-flow restriction, and an accelerator pump. The quantity of fuel in the storage chamber is controlled by a valve actuated by a float. The choke, a butterfly valve, reduces the intake of air and allows a fuel-rich charge to be drawn into the cylinders when a cold engine is started. As the engine warms up, the choke is gradually opened either by hand or automatically by heat- and engine-speed-responsive controllers. The fuel flows out of the idling jet into the intake air as a result of reduced pressure near the partially closed throttle valve. The main fuel jet comes into action when the throttle valve is further open. Then the venturi-shaped air-flow restriction creates a reduced pressure for drawing fuel from the main jet into the air stream at a rate related to the air flow so that a nearly constant fuel-air ratio is obtained. The accelerator pump injects fuel into the inlet air when the throttle is opened suddenly.

Other components have been added to carburetors to reduce hydrocarbon emissions and improve fuel economy. A miniature electronic device called a microprocessor, as part of the engine system, can provide precise control of the intake air-fuel mixture on the basis of engine temperature, load, and speed.

Here are the parts of a carburetor:

-    A carburetor is essentially a tube.
-    There is an adjustable plate across the tube called the throttle plate that controls how much air can flow through the tube.
-    At some point in the tube there is a narrowing, called the venturi, and in this narrowing a vacuum is created.
-    In this narrowing there is a hole, called a jet, that lets the vacuum draw in fuel.

Q1.c)
In addition to the basic components of a carburetor, an Advanced Carburetor is supplied with
i) Air compensating well
ii) Economizer
iii) Accelerating Pump
iv) Double Chamber Carburetor

i) Air Compensating Well:
The reason of providing the compensating well is to introduce some extra air through the air orifice into the carburetor. The will produce a leaner mixture, than that produced by an elementary carburetor.

ii) Economizer:
The economizer is generally provided in the carburetor to supply area amount of fuel under full load (or near full load). In this range of load the carburetor must supply a mixture that will produce maximum power. The economizer enriches the mixture gradually and transforms best economy mixture provided during normal operating of the engine into a best power mixture at full load.

iii) Accelerating Pump:
When the engine speed is increased at a rapid rate (acceleration of a vehicle), the throttle valve is opened sharply. Air accelerates quickly in the carburetor but it takes more time for the fuel to accelerate. Therefore, initially the mixture becomes leaner and the power developed drops. The accelerating pump is a mechanically driven pump which enriches the mixture during acceleration by pumping some extra fuel in the mixing chamber of the carburetor. When the throttle valve operates slowly the pump does not supply any fuel.

iv) Double mixing chamber carburetor (Dual Carburetor):
These carburetors have one common intake pipe, choke valve and float chamber, but two venturi tubes and throttle valves. The accelerating pump and the economizer are generally common to both the chambers. The air from the intake pipe is divided among the two identical mixing tubes containing the venturi sections and fuel from the float chamber enters each venturi tube through separate fuel jet provided with compensating arrangement.

The reason for using two mixing chambers is to effect more uniform mixture strength of charge to different cylinders in case of a multicylinder engine. Each mixing chamber supplies charge to alternate cylinders in the firing order, so that there is practically no overlapping of section periods in one mixing chamber.
 

Q1.d)
Advantages:
The EFI Engine Control Unit (ECU) can control all aspects of the combustion of the air/fuel ratio under all conditions. For each and every load point throughout the rev range that the engine may experience, the EFI ECU can calculate the optimum timing and delivery of fuel as well as precisely when ignition will take place by supplying spark at exactly the right time in order to maximise power and torque. This results in an infinitely more flexible engine with wider power and torque bands.

With EFI, the engine will probably be more reliable, will run smoothly, be more economical (as fuel is not wasted) and have a longer lifespan. As the engine will be more fuel efficient, the combustion of the intake charge is better controlled resulting in fewer emissions, little or no carbon build-up within the engine and of course be much better to drive.

EFI ECUs can actually help prevent engine damage by not allowing full power and torque to be generated until all fluids are up to their normal operating temperature (an anti-thrash mode of sorts!). Nasty pre-ignition/pinging/detonation can be a thing of the past thanks to knock sensors, at the slightest hint of detonation the ECU can slightly alter the ignition timing to fend off potential damage.

In Short, the EFI has following advantages:

1) Increased output per unit of displacement.
2) Lower specific fuel consumption
3) Higher torque at low engine speed.
4) Cleaner exhaust
5) Improve warm up and acceleration.

Disadvantages:
1) EFI conversions are only worthwhile if they are used on vehicles where the ultimate in performance and drivability are desired.
2) It is expensive.
 

Q1.e.
Components of Electronically controlled Carburetor:
The basic carburetor system comprises the following components:
1) Throttle Valve
2) Float System
3) Idle and Transition System
4) Main Metering system and Choke System
5) Sensors

Auxiliary Components and Fuel Control Elements:
1) Throttle valve Actuator is an electro-pneumatic control element for float chamber flow control.
2) Two solenoid valves to which atmospheric and manifold pressures are applied
3) The choke Actuator serves as a final control element for varying the mixture ratio under the various load conditions of operating engine.

Sensors:
There are two types of sensors.
1) The Throttle valve potentiometer detects the position and movement of the throttle valve
2) The temperature sensor detects the engine operating temperature.

Electronic Control Unit ECU:
The control unit receives the signals and process them and calculates the output signals to actuate the throttle and choke actuator.
 
 

Q2a.
COOLING SYSTEM
A cooling system is employed in internal combustion engine to remove heat from the parts heated during operation and to maintain stable thermal condition of the cylinder and piston within the working range of load and speed. Two types of cooling system are in general use, namely water cooling and air cooling system.

WATER COOLING SYSTEM:
Water cooling system is commonly divided into open and closed circulation.

1) Open Circulating System:
Open circulating system uses fresh water from the reservoir or water mains and drains it out after cooling the engine. Therefore, cheap source of cooling water should be available. If there is wide variation between water inlet and outlet temperature it may cause mechanical stress on engine parts. The cooling water needs continuous filtration to keep it free from solids, choking the flow passage.

2) Closed Circulating System:
Closed circulating cooling system, where continuous circulation of cooling water at high flow rate repeatedly cools the engine, is a more perfect cooling system than the open circulation system. The difference between water outlet and inlet temperature is maintained at a smaller value be increasing the flow rate. The hot water coming out after removing heat from the engine is cooled in spray pond, spray tower or radiators mounted on the engines.

In closed circulation system, using a radiator, water is forced through the jacket space of the cylinder block and the cylinder head by a circulating water pump. The hot water leaving the engine enters the radiator. Water flows through tubes provided with external fins. A fan driven by the engine sucks air over the finned tubes and reduce the temperature of the water passing through them. The cooled water leaving the radiator passed through a lubricating oil cooler and finally enters the circulating water pump.

Q2b)
Advantages of field installed radiators:

-    In radiators there are less chance of the coolant loss as it is a closed circuit process.
-    It occupies less space then spray pond.
-    In radiators the coolant can be pumped only by one pump, but we have to need two pumps for open pond cooling.
-    In radiators coolant is safe from impurities than in open pond cooling, where dirt  & etc. can get into  the coolant.
 

Q2c)
 

Q3a)
LUBRICATING SYSTEM:
Internal combustion engine operate at comparatively high speed. When increased in speed after starting the peripheral speed of the journal grows. High peripheral speed causes increased lubricating oil pressure in the journal bearing. The temperatures of the piston assembly and the bearings appreciably grow after continuous operation at high speed. The function of the lubrication system will be to provide oil between bearing parts in order to cool the bearing surfaces by maintaining sufficient rate of oil flow through these surfaces. Lubricating oil also washes out any wear product from journals or other mating parts.

METHODS OF LUBRICATION

i) Oil Bath Lubrication
Oil bath lubrication is widely used method in the case of low or medium speeds. The oil should be at the centre of the lowest rolling element. It is desirable to provide a sight gauge so the proper oil level may be maintained.

ii) Oil Circulation System
Circulating lubrication is commonly used for high speed operation requiring bearing cooling and for bearings used at high temperatures. Oil is supplied by the pipe at the top, it travels through the bearing, and drains out through the pipe an the left. After being cooled in a reservoir, It returns to the bearing through a pump and a filter. The oil discharge pipe should be larger than the supply pipe so that an excessive amount of oil will not back up in the housing

iii) Oil Injection System
Jet lubrication is often used for ultra high speed lubrication such as bearings in jet engines with a dm.n value (dm: pitch diameter of rolling element set in mm; n: rotational speed in rpm) exceeding one million. Lubricating oil is sprayed under pressure from one or more nozzles directly into the bearing. In the diagram the lubricating oil is sprayed on the inner ring and cage guide face. In the case of high speed operation, the air surrounding the bearing rotates with it causing the jet to be deflected. The jetting speed of the oil from the nozzle should be more that 20% of the circumferential speed of the inner ring outer surface

iv) Oil Splash Lubrication
With this lubrication method, oil is splashed onto the bearings by gears or a simple rotating disc installed near bearings without submerging the bearings in oil. It is commonly used in automobile transmission and final drive gears. The figure shows this lubrication method used on a reduction gear

Q3b
WET SUMP SYSTEM:
The lubricating system with a wet sump. The lubricating oil is stored in a pat at the bottom of the crank case. The oil pan is separated from the crank case space by a screen. Oil is delivered under pressure to different sections of the engine by a delivery pump. The parts which are supplied with oil under pressure are main bearing, the crank pin bearing, piston pin bearing, the cam shaft bearing and the valve operating mechanism. Oil is generally sprayed from connecting rod big end to the cylinder liner and the cams. A course filter is in series with the oil line and a fine filter is connected to the oil line in parallel. The course filter usually cleans all the oil supplied to the engine. System with high circulation rate is provided with oil cooler where the oil is cooled by air or water.
 

Q3c:
DRY SUMP SYSTEM:
In this system major part of oil is stored in a tank outside the engine or the crank case. The oil flows down into small recesses at the bottom of crank case. It is continuously pumped out from these recesses into the storage tank by a pump and there is practically no storage of oil in the sump. The delivery pump installed inside the storage tank delivers oil under pressure through oil cooler and filter to different sections of the engine. This system of lubrication is employed only in high power engine and in low height engine.

The oil, after lubricating different parts, flows down into the recesses both in front and rear of crank case. The oil from the recess is continuously pumped out into the storage tank. A separate delivery pump is provided in the oil tank which delivers oil through the oil cooler and filters into the main line. A bypass valve is provided before the oil cooler to return any excess oil delivered by the pump. Usually a drain valve is provided in the main line. This valve ensures constant oil pressure before the bearing irrespective of varying speed and temperature of the engine.
 

Q4a)

One of the more challenging areas to understand concerning the spark-ignition internal combustion engine is that of valve timing.

For every revolution of the camshaft the crankshaft turns twice? That makes sense because the timing gear attached to the camshaft is twice the diameter of the gear on the crankshaft.

At top dead center (TDC), the POWER stroke occurs and the spark plug fires, expanding the combustible mixture in this piston's chamber, moving the piston down to bottom dead center (BDC);
Then, at BDC, the EXHAUST stroke occurs as another piston fires, where the exhaust valve opens for this piston's chamber and the piston moves up to TDC forcing out the exhaust gases;
Then, at TDC, the INTAKE stroke occurs as another piston fires, where the intake valve opens and a combustible mixture is sucked into this piston's chamber as the piston moves to BDC;
Finally, at BDC, as another piston fires, with both the intake and exhaust valves closed for this piston's chamber, the COMPRESSION stroke occurs where the piston moves up to TDC and the combustible mixture is compressed...

The above representation isn't exactly right. We know that the valves cannot instantly open when they should be open, nor close when they should be closed... They have some inherent opening and closing time... For example, to efficiently push out exhaust, we need to open the exhaust valve some time before the power stroke is complete and the piston reaches bottom dead center (BDC) so it is open long enough to allow the spent gases be pushed out during the entire exhaust stroke... Likewise, we probably need to have the intake valve open sometime during the exhaust stroke to ensure efficient cylinder filling during the intake stroke... That means the exhaust valve and the intake valve will be open at the same time, with some OVERLAP... And that also means, if we want to maximize our flow efficiencies, that the exhaust valve will still be open some time during the intake stroke, and the intake valve has yet to close when the compression stroke has begun.

Now we have graphical representations of what we have been discussing, both on a crankshaft degree/cam lift phase diagram (linear) and a crankshaft degree circle diagram... We can see what the durations and overlap are, and see the difference between advertised and 0.050" lift durations, etc. But again, what are the EVO, IVO, EVC, and IVC terms?

Let's define some of the terminology given in the graph above:

Advertised Duration - the number of crankshaft degrees an intake or exhaust valve is open, where open is often considered any cam lobe lift greater than 0.005";
Duration at 0.050" valve lift - the number of crankshaft degrees an intake or exhaust valve is open, where open is considered any valve lift greater than 0.050". This is often used for duration numbers when specifying a cam, because it is generally accepted that "meaningful flow" occurs at this lift. (Note: For many Chrysler cams, an approximate duration at 0.050" valve lift can be found by multiplying the advertised duration by 0.850. This is profile dependent though, and again, only approximate);
Centerline - the point of maximum lift on a cam lobe;
Dual pattern - a camshaft that has different profiles on the intake and exhaust lobes;
Single pattern - a camshaft that has identical profiles on the intake and exhaust lobes;
Lobe separation angle (a.k.a. lobe center) - the angle measured in camshaft degrees between the point of maximum lift on the intake lobe and the point of maximum lift on the companion exhaust lobe;
Overlap - the number of crankshaft degrees the intake and exhaust valves for the same cylinder are open;
EVO (Exhaust Valve Opening duration) - the number of crankshaft degrees before bottom dead center, during the power stroke, that the exhaust valve is open;
IVO (Intake Valve Opening duration) - the number of crankshaft degrees before top dead center, during the exhaust stroke, that the intake valve is open;
EVC (Exhaust Valve Closing duration) - the number of crankshaft degrees after top dead center, during the intake stroke, that the exhaust valve is open;
 

IVC (Intake Valve Closing duration) - the number of crankshaft degrees after bottom dead center, during the compression stroke, that the intake valve is open;

Q4b)
The overlap phase, which is very critical to vacuum, throttle response, emissions and especially, gas mileage. The amount of overlap, or the area between the intake opening and the exhaust closing, and where it occurs, is one of the most critical points in the engine cycle. If the intake valve opens too early, it will push the new charge into the intake manifold. If it occurs too late, it will lean out the cylinder and greatly hinder the performance of the engine. If the exhaust valve closes too early it will trap some of the spent gases in the combustion chamber, and if it closes too late it will over-scavenge the chamber; taking out too much of the charge, again creating an artificially lean condition. If the overlap phase occurs too early, it will create an overly rich condition in the exhaust port, severely hurting the gas mileage. So, as you can see, everything about overlap is critical to the performance of the engine.
 
 

Q5a)
POLLUTION:
All the I.C. engine when they operate pollute the environment through hot combustion gases. The automotive vehicle and industrial power unit using internal combustion engine are major contributors to this problem.

Internal combustion engine, using external mixture formation has sources of emmittants. These are emittants from carburetor and fuel tank vents, crank case vents and engine exhaust. The emission from first two sources can be easily collected, stored and subsequently burnt in the engine combustion chamber. Therefore, the main source of emission is the exhaust from engine. The fuels used in the engine are mainly refined petroleum products containing hydrocarbons with a trace of sulphur. If combustion is complete inside the cylinder, the exhaust will be a mixture of carbon dioxide, water vapour, nitrogen, oxygen (due to excess air) and traces of sulphur dioxide. None of these products has toxicity. Also when combustion is complete (usually with excess air) no smoke or particulate are produced.

Toxic effect and smoke or particulate formation being the main cause for pollution, these products are not considered as pollutants. But combustion process taking place in internal combustion engine is never perfect or complete. As a result some products are formed having undesirable properties. These products are considered to be pollutants when they come out during exhaust process.

The following products are usually considered as pollutants.

a) Carbon monoxide:
Carbon monoxide is generally formed when the mixture is rich in fuel. The amount of carbon monoxide formed increased as the mixture becomes more and more rich in fuel.

b) Nitric Oxide:
The rate of formation of nitric oxide is higher with rich mixture than with lean mixture. Nitric oxide does not decompose sufficient during expansion. As a result, nitric oxide concentration in exhaust will be high at full load when the fuel air ratio is high. The Nitric oxide are harmful for biological life and must be controlled by adapting different method of reduction of Nitric oxide.

c) Hydrocarbons:
Complex chain of hydrocarbons result in polymerization and agglomeration during combustion and some of hydrocarbon escape into exhaust due to imperfect combustion.

d) Smoke or particulate:
Particulate matter (matters which cause visible smoky exhaust) generally occurs in liquid or solid form. The solid particles are usually formed by dehydrogenation (taking out hydrogen molecules from the hydrocarbon), polymerization (forming of higher molecular weight hydrocarbon) and agglomeration (combining of different molecules).

e) Sulphur Oxide:
The oxides of sulphur are formed during combustion. These oxides are harmful for the engine itself as well as general industry and life.

Pollutants and their Harmful Effects:

Photochemical Smog: Some hydrocarbons and oxides of nitrogen present in the exhaust react with atmospheric air in the presence of sunlight and produce photochemical smog. This smog produces undesirable effects on human health, plants and some materials. It produces eye irritation and affects the respiratory system of human being. It has damaging effect on plant life. Sulphur dioxide emitted from engine exhaust is small in quantity. It may combine with fog to produce smog. The smog produces has undesirable effect on human system.

Odour: Odour emitted from SI engine is small and non-irritating. But the odour emitted from diesel engine, specially from bus or truck engine, is sometimes pungent and irritating.

Toxicity: Carbon monoxide has toxic effect on human being. The oxides of nitrogen present in atmosphere also have effect similar to that of carbon monoxide. A part of them settle on the haemoglobins in blood. This results in less amount of oxygen carrying haemoglobins and effect the blood purifying process.
 
 

Q5b)
METHODS OF POLLUTANT DETECTION:

Flame Ionization Detector:
Flame produced by burning pure oxygen and hydrogen is practically from from ions. Addition of inert gases with fuel (hydrogen oxygen mixture) does not affect ion production. If hydrocarbon is introduced with the mixture even in minute quantities, sufficient amount of ion will be formed in the products. Under suitable conditions, ion production will be proportional to the amount of hydro carbon present. This principle is used in flame ionization detector.

The basic elements of the detector are a burner and an ion collector. Exhaust sample is mixed with hydrogen diluted with helium in the burner. The mixture is burned in diffusion flame with air. Two collector plates are connected to two terminals of battery. The ions move to the negatively charged collector. Electrons travel from positively charged collector to the negatively charged one. Thus a small current flows between collector plats. This current is proportional to the ions formed or to the amount of hydrocarbon present.

Non-dispersive infrared gas analyzer:
Some of the emission gases, like carbon dioxide, carbon monoxide and nitric oxide, absorb infrared energy of particular wave length bands when it passes through them. The instruments used for emission measurement are based on the differential absorption of infrared energy from two gas cells, one containing the exhaust sample and the other containing non-absorbing gas.

Infrared energy falls on the sample cell as well as the reference cell from one end and the detector placed on the other end of the cell, measures the energy of the particular band of wavelengths. The source supply a constant infrared energy input to both the gas cells. The difference of the amount of energy transmitted from the cell set a particular wave band gives the concentration of the component in the gas sample.

Wet Chemical analysis for measurement of aldehydes:
This method is based on the reaction of formaldehyde with chromotropic acid resulting in a measurable colour. A known volume of exhaust gas is bubbled through a 0.1 percent solution of chromotropic acid in sulphuric acid. A stable colour suitable for spectophotometric measurement at a suitable light wavelength is produced when the solution is heated for about 30 minutes at 100^oC. When large amount of aromatic hydrocarbon is present in the sample the interference can be reduce by using 1 percent sodium bisulphate which will absorb the aromatic compounds.

MBTH Method:
This method is based on the reaction of aldehydes with MBTH (3-methyl-2-benzothizolone hydrazone) in the presence of an oxidising agent, producing a blue dye. A known volume of exhaust gas is passed through 10 ml of 0.4 percent aqueous solution of MBTH chloride in a container. After one hour the solution is mixed with 25ml of 1 percent solution of ferric chloride in water in a 100 ml flask. After another waiting period of about 20 minutes the flask is filled with acetone. The absorbance of the solution at a particular wavelength of light is measured by spectrophotometric measurement. Aldehyde concentration can be read from the calibration curve.
 
 

Q5c)
THE CATALYTIC CONVERTER
When your engine burns fuel, it produces gases that are bad for the environment. These noxious gases are hydrocarbons, carbon monoxide and nitrogen oxides. To prevent the engine from polluting the environment with these gases, we include a catalytic converter in our emission systems.

The catalytic converter is installed in the exhaust line, between the exhaust manifold and the muffler, and makes use of chemicals that act as a catalyst. A catalyst is a chemical that causes a reaction between other chemicals without being affected itself. In the case of the catalytic converter, the chemicals it contains cause a reaction in the pollutants in the exhaust. The pollutants are changed from harmful gases to harmless ones before they are let into the environment through the tail pipe.

Basically, the harmful gases enter the catalytic converter, a kind of stainless steel container. The converter is lined with chemicals such as aluminum oxide, platinum and palladium. These chemicals cause the carbon monoxide and hydrocarbons to change into water vapor and carbon dioxide. Some converters have a third lining of chemicals, platinum and rhodium, that reduce nitrogen oxides (three-way, dualbed converter).

Problems with catalysts involve their intolerance for leaded fuels and the need to prevent overheating.

The reason that leaded gas cannot be used in an engine with a catalytic converter is that the lead coats the chemicals in the converter. This makes them unable to do the job anymore, since the chemical lining can't come in contact with the pollutants. At first, this was a big disappointment, because lead acted as a lubricant and helped to reduce wear on some of the engine parts. Luckily for our engines and the environment (not to mention us), car manufacturers soon got around the problem by making tougher parts and coating them with special metal.

Q5d)
Exhaust-gas recirculation is a technique to control oxides of nitrogen, which are formed by the chemical reaction of nitrogen and oxygen at high temperatures during combustion. Either reducing the concentrations of these elements or lowering peak cycle temperatures will reduce the amount of nitrogen oxides produced. To achieve this, exhaust gas is piped from the exhaust manifold to the intake manifold. This dilutes the incoming fuel-air mixture and effectively lowers combustion temperature. The amount of recirculation is a function of throttle position but averages about 2 percent.

Manifold reactors are enlarged, insulated exhaust manifolds into which air is injected and in which exhaust gas continues to burn. The effectiveness of such units depends on the amount of heat generated and the length of time the gas is within the manifold. Stainless steel and ceramic materials are used to provide durability at high operating temperatures (approaching 2,300 F [about 1,300 C]).