13-01-2011, 04:32 PM
[attachment=8193]
Guided by,
Prof.V.M. KORDE
Submitted By:
Anup.V.Wakodikar
CONTENTS
Introduction to Engines
History Of HEMI Engines
Construction of HEMI Engines
Working of HEMI Engines
Comparison between HEMI and Flathead Engine
Advantages
Disadvantages
Modern users of HEMI
HISTORY OF “HEMI” ENGINES
6-Cylinder HEMI Engine was found by Harry Westlake in 1948
180 Horsepower HEMI V-8 Engine was found by Chrysler in 1951(“331 HEMI”)
In 1956 Chrysler reintroduced modified HEMI engine having displacement of 354 cubic inch
In 1957 Chrysler again introduced a HEMI engine having displacement of 392 cubic inch
In 1964 Chrysler introduced a 7-liter version having displacement of 426 cubic inch
In 1965 the 426 Street HEMI came out producing 425 horsepower
In 2002 Chrysler introduced HEMI Engine having valve and twin spark plug
CONSTRUCTION OF HEMI ENGINES
1.VALVE TRAIN:-The inlet and exhaust valves are located on opposite sides of the chamber at 90 degrees to the cylinder bank.
2.CYLINDER HEAD:-The cylinder head are shaped spherically, like the inside of a half cut ball. This allows for a symmetrical placement of bigger valves. the intake and exhaust valves are usually on opposite sides of the chamber, allowing for the combustion mixture to flow directly across the chamber, commonly referred to as "cross-flow" .The air-fuel mixture flows
through a Hemi chamber in a
straighter, less restrictive manner
than in a wedge-type chamber.
More mixture can also enter the
chamber for faster ignition, which
allows the engine to produce more
horsepower.
3.PISTONS:-All original pistons are cast
aluminium alloy. Most have slight
compression domes (area of slightly
smaller diameter than the bore)
4.CONNECTING ROD:-Each engine has
its own unique rod. All rods are high
quality forged steel, and more than
adequate in strength for the original
power produced.
5.CRANKSHAFT:-All crankshafts are high quality forged steel with rolled fillets, and use an un-threaded 8 bolt (obsolete pattern) flange.
WORKING OF HEMI ENGINES
Working of HEMI Engine is based on Otto cycle which consist of the following strokes. A stroke refers to the full travel of the piston from Top Dead Center (TDC) to Bottom Dead Center (BDC).
1.Intake Stroke:-On the intake or induction stroke of the piston , the piston descends from the top of the cylinder to the bottom of the cylinder, reducing the pressure inside the cylinder. A mixture of fuel and air is forced by atmospheric (or greater) pressure into the cylinder through the intake port. The intake valve(s) then close.
2.Compression stroke:-With both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the fuel-air mixture. This is known as the compression stroke.
3.Power stroke:-While the piston is close to Top Dead Center, the compressed air–fuel mixture is ignited, usually by a spark plug (for a gasoline or Otto cycle engine) or by the heat and pressure of compression (for a diesel cycle or compression ignition engine). The resulting massive pressure from the combustion of the compressed fuel-air mixture drives the piston back down toward bottom dead center with tremendous force. This is known as the power stroke, which is the main source of the engine's torque and power.
4.Exhaust stroke:-During the exhaust stroke, the piston once again returns to top dead center while the exhaust valve is open. This action evacuates the products of combustion from the cylinder by pushing the spent fuel-air mixture through the exhaust valve(s).
The intake (A) stroke is performed by an isobaric expansion, followed by the compression (B) stroke, performed by an adiabatic compression. Through the combustion of fuel an isochoric process is produced, followed by an adiabatic expansion, characterizing the power © stroke. The cycle is closed by an isochoric process and an isobaric compression, characterizing the exhaust (D) stroke.
ADVATAGES OF HEMI ENGINES
In a HEMI engine, valves can be large so the airflow through the engine is improved.
The air-fuel mixture flows through a Hemi chamber in a straighter, less restrictive manner
With the hemispherical combustion chamber design, the intake and exhaust valves are usually on opposite sides of the chamber, allowing for the combustion mixture to flow directly across the chamber
In a spherical design, the size of the valves can be much larger. The larger the valves, the larger their corresponding openings during the 4-stroke combustion process. That translates into more airflow.
A HEMI engine has less surface area than a conventional flat head engine. That means the fuel within the chamber burns more efficiently because less heat is lost to the limited surface area.
DISADVANTAGES OF HEMI ENGINES
The main drawback to a HEMI engine is the number of valves it allows per cylinder. Because the top of the chamber is shaped like a sphere, it can only fit two valves
Expensive to produce
The pistons are heavy (high reciprocating mass) and the large dome shrouded spark plugs and valves, requiring more ignition timing and increasing hydrocarbon emissions
