CONTENTS Introduction to Engines

Types of Engine

Construction of HEMI Engines

Different Types Of Engines

Parts Of Hemi Engine

Working of HEMI Engines

Comparison between HEMI and Flathead Engine

Benefits and Drawbacks

IntroductionHemi Heads require a more complex

valvetrain, including a "Double Rocker" system which uses two rocker shafts per head.

The extra parts, complexity and flexibility tend to limit RPMs. Adding to the cost of production, the piston castings are more complex as well.

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, commonly referred to as "cross-flow" heads.

Types of Engines4 Stroke

Flat HeadVHemi

2 stroke engineFlat

Rotary

RocketDiesel

2 stroke 4 stroke

Jet or Turbine

Different types Of Hemi Engines

5.7-liter HEMI Magnum V-8 engine

Parts Of Hemi EngineCylinder

The core of the engine is the cylinder. The piston moves up and down inside the cylinder. The engine described here has one cylinder. That is typical of most lawn mowers, but most cars have more than one cylinder (four, six and eight cylinders are common). In a multi-cylinder engine the cylinders usually are arranged in one of three ways: inline, V or flat (also known as horizontally opposed or boxer).

Spark PlugThe spark plug supplies the spark that ignites

the air/fuel mixture so that combustion can occur. The spark must happen at just the right moment for things to work properly.

ValvesThe intake and exhaust valves open at the

proper time to let in air and fuel and to let out exhaust. Note that both valves are closed during compression and combustion so that the combustion chamber is sealed.

PistonA piston is a cylindrical piece of metal that

moves up and down inside the cylinder.

Piston ringPiston rings provide a sliding seal between the

outer edge of the piston and the inner edge of the cylinder. The rings serve two purposes:

They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion.

They keep oil in the sump from leaking into the combustion area, where it would be burned and lost.

Combustion ChamberThe combustion chamber is the area

where compression and combustion take place. As the piston moves up and down, you can see that the size of the combustion chamber changes. It has some maximum volume as well as a minimum volume. The difference between the maximum and minimum is called the displacement and is measured in liters or CCs (Cubic Centimeters, where 1,000 cubic centimeters equals a liter).

Connecting RodThe connecting rod connects the piston to the

crankshaft. It can rotate at both ends so that its angle can change as the piston moves and the crankshaft rotates.

ExhaustOnce the piston hits the bottom of its stroke,

the exhaust valve opens and the exhaust leaves the cylinder to go out the tail pipe.

SumpThe sump surrounds the crankshaft. It

contains some amount of oil, which collects in the bottom of the sump (the oil pan).

CompressionThen the piston moves back up to compress

this fuel/air mixture. Compression makes the explosion more powerful.

Intake StrokeThe piston starts at the top, the intake valve

opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work.

CombustionWhen the piston reaches the top of its stroke,

the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down

Crankshaft The crank shaft turns the piston's up and down motion into

circular motion just like a crank on a jack-in-the-box does.

Working Of Hemi EngineWorking 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 (C) stroke. The cycle is closed by an isochoric process and an isobaric compression, characterizing the exhaust (D) stroke.

OTTO CYCLE

The Otto cycle p-V diagram

COMPARISON BETWEEN HEMI AND FLATHEAD ENGINE

• In a HEMI engine, the top of the combustion chamber is hemispherical, as seen in the image above

• In a Flathead engine, the top of the combustion chamber is flat, as seen in image above

• The intake and exhaust valves are usually on opposite sides of the chamber

• The valves are in the block, rather than in the head, and they open in a chamber beside the piston

• Thermal efficiency in HEMI engine is extremely good

• Thermal efficiency in flathead engine is poor

• HEMI engines are costly engines

• Flathead engines are cheap engines

Benefits and drawbacks

Although a wedge-head design offers simplified valve actuation, it usually requires the air/fuel mixture to make sharp turns en route to and from the chamber.

With a hemispherical chamber, larger valves are possible and a straighter, less restrictive flow path can be provided for the air/fuel mixture.

This improves engine breathing. Placing the spark plug near the center of the chamber aids in achieving complete combustion of the fuel/air mixture, though it is not mandatory.