Physics Report on a Visit to the Royal Naval Engineer College

Britannia Royal Naval College Visit Report

Introduction

On the 28th March 2006, our physics group visited the Dartmouth Royal Naval Engineer College where we found out more about engines and materials, connecting to several aspects of physics.

One of the aspects is the energy accounting of diesel engines for the ships. The engine needs power to operate and it gets that from the fuel, the efficiency of the engine can tell us how much power is used to heat up the engine, and how much power is wasted as noise our shaking of the engine.

Efficiency (in %) =

Another important aspect is the mechanics of materials and how they can be tested for making metal cables. As in order to quantify the effects of various types of loads on a material it’s often necessary to know the mechanical properties of the material. These properties are often determined from standardised tests.

Finding out the stress, strain and Young’s Modulus is very important as they quantify the tensile strength of a material, playing a big part in making cables.

When we apply a force to a material, it’s being subjected to stress ().

When a material is subjected to a tensile or compressive force it changes in length; strain () is the change in length per unit length.

When stress and strain are proportional they are obeying Hooke’s Law.

Tension ∝ extension or Stress ∝ strain

The Elastic Constant or Youngs Modulus (E) is related to stress and strain as follows:

Young’s modulus is a fundamental property of a material which describes its stiffness, the ability to maintain its shape. A high Young’s modulus value indicates that the material will not stretch by much under load.

A Tensile Test is carried out to determine the properties, where a force is applied to a material of known length and uniform shape, in gradual increment noting the extension, the test load is applied until the material breaks.

Tensile strength
Yield stress
Youngs Modulus

The Engine

Diesel engines are used extensively in the navy, serving as propulsion units for small boats, ships and land vehicles, so it’s important to know how the engine exactly works.

Here is a simplified version of a diesel engine:

Diesel engine is a type of internal combustion engine, it involves the combustion of a suitable fuel inside a cylinder containing a piston, whose motion results from the transformation of thermal energy into mechanical work.

More specifically, it is a compression ignition engine, in which the fuel is ignited by being suddenly exposed to the high temperature and pressure of a compressed gas, rather than by a separate source of ignition, such as a spark ...

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Here is a simplified version of a diesel engine:

The moving parts of the diesel engine provide for controlling the elements necessary for combustion and the transformation of combustion to mechanical shaft energy.

When a gas is compressed, its pressure rises, obeying the combined gas law.

A diesel engine uses this property to ignite the fuel. Air is drawn into the cylinder of a diesel engine and compressed by the rising piston. At the top of the piston stroke, diesel fuel is injected into the combustion chamber at high pressure, through an atomising nozzle, mixing with the hot, high-pressure air. The resulting mixture ignites and burns very rapidly. This contained explosion causes the gas in the chamber to heat up rapidly, which increases its pressure, which in turn forces the piston downwards. The connecting rod transmits this motion to the crankshaft, which is forced to turn, delivering rotary power at the output end of the crankshaft.

The first marine diesel engine produced was a four-stroke engine, which has replaced anything invented before with its high efficiency.

The four strokes of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston; therefore the complete cycle requires two revolutions of the crankshaft to complete.

Two of the more common small boat engines used in the Navy today are the 6-71 General Motors Detroit Diesel engine and the Westerbeke Model 4-107. The reason for their popularity is that they are reliable and parts are easy to obtain.

Diesel engines are also used as prime movers in auxiliary machinery in the navy. But the other major uses are for vehicles, aircrafts and large-scale portable power generators.

Diesel engines produces useful work to drive ships and other electrical generators, it’s useful to know how powerful the engine is, in another word how quickly the engine use up energy, which is measured in joule J or Newton metres Nm.

The three types of energy which are important in engines are-

Heat
Electrical
torque

They will enable us to find out how power, measured in Watt W or Joule per second Js-1 is used and wasted in a ship engine.

To find out how an engine uses and loses power, and the efficiency of the engine, we performed some tests on a small diesel engine which is fitted with measuring instruments.

Fuel power- how much power the engine gets from burning the fuel

To find out MFR (mass flow rate) or the mass of fuel burnt per sec, a measuring cylinder and a timer is attached to the engine.

Timer gives the time, which was 260s, engine speed and number of revolutions of the engine for the period during which a set amount of fuel is used by the engine.
The cylinder measures the volume of fuel used, which was100ml, or 0.0001m3

MFR (kg s-1) = Volume (ml) × density (kgm-3) ÷ time (s)

So to find the fuel power, we multiply the MFR by the calorific value (known)

Power from the fuel = mass of fuel per sec × heat per kg of fuel (calorific value)

Pfuel = kg s-1 × J kg-1

0.0001m3 × 835 kgm-3 ÷ 260s × 4.526*107 J kg-1 = 14535 Js-1

Brake power- how much power that comes out of the engine to do work

It’s measured using a dynamometer, which is an electrical generator. When the electricity is generated the outer casing (stator) tries to turn with the rotor, a lever and spring force balance prevent the stator from moving too far.

The force balance measures how much force the level is using, which is the torque (rotary force).

Torque (Nm) = Force (F) × Radius (m)

Using torque and the engine speed, brake power is calculated:

Brake power = 2π ÷ 60 × revolutions per second × torque

=0.105 × RPM × torque

0.105 × 2008 s-1 × 79N × 0.22m = 3664 Js-1

Exhaust Power-

Calculated by finding the amount of exhaust coming out of the engine, or the Mass Flow Rate of air going into the engine

Plenum Chamber is a large drum with a hole in one end and a tube going from the side into the engine. Air flows through an orifice in a plate into the drum and goes to the engine. A pressure gauge measures the difference in air pressure between inside and outside of the drum, which enables us to find the flow rate values.

The amount of heat in the exhaust also depends on the temperature of air going in and coming out, so we need to find the temperature difference of the two gases, which is read from a temperature gauge.

Power = Mass flow rate (kg s-1) × temp difference (K) × heat capacity (J Kg-1K-1)

0.0095 kg s-1 × 278.2 K × 1010 J Kg-1K-1 = 2669 Js-1

Power in the cooling water

Power is given to the water when it takes heat energy away from the engine to stop it from seizing up. The flow rate of water is read from the water flow meter.

Power = Mass flow rate × temp difference × heat capacity of water

0.023 kg s-1 × 2.3 K × 4190 J Kg-1K-1 = 225 Js-1

The engine needs power to run itself and it gets that from the fuel. We are interested in how much power is wasted in heating up the engine, noise, and shaking of the engine.

From the tests we did on the small diesel engine, we found that:

The efficiency of any process which converts something from one form to another is:

Efficiency (in %) =

To find the efficiency of the diesel engine:

= = 25.21%

From this, we can see that the engine is not very efficient, power is still lost to exhaust and to cooling water, even though diesel engines are more efficient that normal gasoline engines.

On the environmental side, diesel engines produce very little carbon monoxide compared with gasoline engines as they burn the fuel in excess air. However, they can produce black soot from their exhaust, consisting of unburned carbon compounds which is often caused by worn injectors that do not atomize the fuel sufficiently. This have been implicated in health problems. Therefore most modern diesel engines catch the soot in a particle filter. Other problems associated with the exhaust gases (nitrogen oxides, sulfur oxides) can be mitigated in future with further investment and equipment.

In the future, the engine’s efficiency (fuel economy) can be maximised, and the emissions can be minimised by controlling the timing of the start of injection of fuel into the cylinder.

Also the addition of a turbocharger or supercharger to the engine would greatly assists in increasing the efficiency and power output.The increased fuel economy of the diesel over the petrol engine would mean that the diesel produces less carbon dioxide (CO2) per unit distance.

It is hoped that as hybrid electric cars are becoming more common, the Navy will be able to bring hybrid electric ships to the high seas. The Office of Naval Research (ONR) is developing innovative propulsion systems based on new fuel-cell technology for efficient generation of electrical power and greater design flexibility for future ships.

Unlike gas turbines and diesel engines, fuel cells do not require combustion, and therefore do not produce pollutants such as nitrogen oxide. Fuel cells are also far more efficient than combustion engines. The fuel cell system that is developing will be capable of between 37-52%efficiency. Moreover, fuel cells can be dispersed throughout the ship instead of being co-located with the ship’s shaft. This added flexibility will improve ship survivability.

Reforming diesel is especially tricky due to the sulfur present in the fuel. The integrated fuel processor heats and vaporizes the diesel, and then the sulfur in it is converted into hydrogen sulfide. The hydrogen sulfide is then exposed to zinc oxide, oxidizing the sulfur into sulfur dioxide and separating it from the hydrogen.

Fuel cells combine improved efficiency, low emissions and design flexibility, all of which help slash shipbuilding costs.

References:

“Physics in Action” booklet by Department of Engineering Science of Britannia Royal Naval College Dartmouth

http://www.keveney.com/otto.html

http://www.news.navy.mil/search/display.asp?story_id=12221

http://www.fas.org/man/dod-101/sys/ship/eng/diesel.htm