On the World Environmental Day, I find Mankind in a very crucial situation. With all its development engines running heavily on fossil fuels, we are at the most difficult juncture.
Frankly speaking – the writing was always bold on the wall – after all how long one can manage with one’s ancestral bank balance? Reserve of fossil fuel was always limited. And everyone knew that it is not replenishable. It was only a matter of time before it had to happen. And it happened – the sky rocketed crude price has slowed the growth engine. Without any practical and affordable alternative, we are left out with one option – CONSERVE.
One might ask – why we were silent all these days? My assumption is – it was easier that way.
Let me take the Automotive example – one of the double edged swords - one hand depleting oil reserves and global warming on the other. And let me take a more specific area – Indian mass and goods transport. In current scenario, with booming economy and nicely laid roads, road transport is the choice of the individual and industry.
To understand the basics – fuel is burnt in automotive engines to generate momentum. That is, if the weight remains the same, it is used to accelerate from one speed to another higher speed. Then, why do we need fuel to maintain a constant speed?
As we all know, the world in not perfect – there are many resistances and losses we need to overcome just to maintain constant speed. Given a vehicle, two major such resistances are “Rolling Friction” and “Aerodynamic Drag”. The first one is commonly taken care by keeping the vehicle weight minimum, regularly checking and correcting wheel tube air pressure. My concern is the second one – which has more potential and generally neglected.
Aerodynamic drag is the resistance offered by atmospheric air to a moving object. Though air is not seen, it has all characteristic of a fluid. The resistance has two reasons –
1. The friction between air and the skin of the object
2. The resistive thrust generated due to difference in pressure between the front and rear of the object
And the second reason is more contributing.
The drag (or the resistance) is proportional to three things –
1. Cross sectional area of the object (A)
2. Air density (d)
3. Square of object velocity (V)
To get the absolute figure, this product is to be multiplied by another factor – Cd – coefficient of Drag, which is dependant on the shape
Drag = Cd x A x (1/2 x d x V x V)
If one closely observes, velocity is a crucial factor – 10% increase in velocity means 21% increase in drag. At higher speed, drag overtakes rolling friction easily. That means – merely checking tyre pressure will not help at higher speeds. And, to add to our concern, in India, average transportation speed has increased – thanks to better roads and improved vehicle. Ask any one in logistics sector – travel time has come down to half in many cases. And there is my concern.
Another closer look reveals - nothing is left to the transport owner or the driver. Cross sectional area is fixed for a vehicle and the air density depends on the operating altitude and temperature. The economics of the business does not allow the driver to bring down the speed – it does not help the country’s economy either. So, we are left with the main factor – Cd.
To understand the Impact of Cd, let us take some common shapes. Two extreme cases could be – Smooth Sphere - 0.1 and Smooth brick - 2.1
It is a common sense that a smooth flowing shape means lower Cd, hence low resistance. Based on this theory, through ages, people are interested in making smooth shapes to reduce resistance and attain speed. This is more relevant in Aerospace Industry. Boering 747 has Cd of 0.031. In automotive sector, in search of speed and efficiency, people have attained new heights. There are commercial cars in today’s world with Cd as low as 0.3. For Concept and Racing cars, this figure is really low.
Here comes my point – what would be the Cd of our trucks and buses? I always surprised by the "blunt, square shapes" of our mass transport vehicles. In no imagination that could be Aerodynamic. I do not have any figure but am sure it will be considerably higher than the desired value. As I tried to explain, drag may not help us at low speeds or “stop and go” scenario. But for highway driving at high speeds it definitely matters. That means, for long distance, intercity buses and trucks, it has to be a factor to be considered for fuel economy.
Frankly speaking – the writing was always bold on the wall – after all how long one can manage with one’s ancestral bank balance? Reserve of fossil fuel was always limited. And everyone knew that it is not replenishable. It was only a matter of time before it had to happen. And it happened – the sky rocketed crude price has slowed the growth engine. Without any practical and affordable alternative, we are left out with one option – CONSERVE.
One might ask – why we were silent all these days? My assumption is – it was easier that way.
Let me take the Automotive example – one of the double edged swords - one hand depleting oil reserves and global warming on the other. And let me take a more specific area – Indian mass and goods transport. In current scenario, with booming economy and nicely laid roads, road transport is the choice of the individual and industry.
To understand the basics – fuel is burnt in automotive engines to generate momentum. That is, if the weight remains the same, it is used to accelerate from one speed to another higher speed. Then, why do we need fuel to maintain a constant speed?
As we all know, the world in not perfect – there are many resistances and losses we need to overcome just to maintain constant speed. Given a vehicle, two major such resistances are “Rolling Friction” and “Aerodynamic Drag”. The first one is commonly taken care by keeping the vehicle weight minimum, regularly checking and correcting wheel tube air pressure. My concern is the second one – which has more potential and generally neglected.
Aerodynamic drag is the resistance offered by atmospheric air to a moving object. Though air is not seen, it has all characteristic of a fluid. The resistance has two reasons –
1. The friction between air and the skin of the object
2. The resistive thrust generated due to difference in pressure between the front and rear of the object
And the second reason is more contributing.
The drag (or the resistance) is proportional to three things –
1. Cross sectional area of the object (A)
2. Air density (d)
3. Square of object velocity (V)
To get the absolute figure, this product is to be multiplied by another factor – Cd – coefficient of Drag, which is dependant on the shape
Drag = Cd x A x (1/2 x d x V x V)
If one closely observes, velocity is a crucial factor – 10% increase in velocity means 21% increase in drag. At higher speed, drag overtakes rolling friction easily. That means – merely checking tyre pressure will not help at higher speeds. And, to add to our concern, in India, average transportation speed has increased – thanks to better roads and improved vehicle. Ask any one in logistics sector – travel time has come down to half in many cases. And there is my concern.
Another closer look reveals - nothing is left to the transport owner or the driver. Cross sectional area is fixed for a vehicle and the air density depends on the operating altitude and temperature. The economics of the business does not allow the driver to bring down the speed – it does not help the country’s economy either. So, we are left with the main factor – Cd.
To understand the Impact of Cd, let us take some common shapes. Two extreme cases could be – Smooth Sphere - 0.1 and Smooth brick - 2.1
It is a common sense that a smooth flowing shape means lower Cd, hence low resistance. Based on this theory, through ages, people are interested in making smooth shapes to reduce resistance and attain speed. This is more relevant in Aerospace Industry. Boering 747 has Cd of 0.031. In automotive sector, in search of speed and efficiency, people have attained new heights. There are commercial cars in today’s world with Cd as low as 0.3. For Concept and Racing cars, this figure is really low.
Here comes my point – what would be the Cd of our trucks and buses? I always surprised by the "blunt, square shapes" of our mass transport vehicles. In no imagination that could be Aerodynamic. I do not have any figure but am sure it will be considerably higher than the desired value. As I tried to explain, drag may not help us at low speeds or “stop and go” scenario. But for highway driving at high speeds it definitely matters. That means, for long distance, intercity buses and trucks, it has to be a factor to be considered for fuel economy.
Can someone tell me the Cd figure for our buses and trucks? I would love to see one of our institutes taking this up as a project and come up with some suggestions. And I believe the savings could be as significant as 10%! Can any one substantiate?
Inputs from –
Reducing Aerodynamic Drag and Fuel Consumption - Fred Browand
Inputs from –
Reducing Aerodynamic Drag and Fuel Consumption - Fred Browand
http://www.edmunds.com/advice/fueleconomy/articles/106954/article.html
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