Inspiration:
In reality, I do believe we have family guy and housewife who would spend time with their children. By considering mass of the car alone is not good enough to save fuel for their case.One more factor, they need to concern is the ENGINE SIZE of the car as it is related to the engine's maximum power output of the car as shown in Chart 1 below. For example, when you drove your car with full passenger capacity, your car gained weight, so is your car's engine is powerful enough to handle this loading?
Simple illustration, you drive your car with 4 other passengers in Suzuki Swift. You could feel your car will be a lot weaker than you were driving alone especially you were going up a slope that against gravity.
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| Chart 1: The relationship between engine size and maximum power output of an engine for 7 selected Honda Models. |
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| Chart 2: The relationship between engine size and vehicle mass based on 7 Honda Models |
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| Chart 3: The relationship between JC08 fuel consumption rating and vehicle mass based on 7 Honda Models |
As seen above,
Chart 2 showed that it was an upward trend that heavier the vehicle tend to have greater engine size except one model (which is the outlier)
Chart 3 supported my statement in the previous topic that lighter the vehicle tend to have better fuel economy but the outlier in Chart 2 is the same outlier in Chart 3.
Physics 101:
If you read the previous topics, you should be very familiar with the terminology of FORCE and WORK. If you didn't, either you read them again or just a quick revision.By Newton's Second Law,
Force = mass x acceleration [unit: N]
where k defined 1 Newton Force as amount of Force to increase the 1m/s of speed of an object with 1 kg mass for each seconds.
Work Done = Force x Distance (travelled along the force acted) [unit: J]
New terminology introduced here:
Power is the rate of work done/work output.
In short, power is the amount of energy released every second. [unit: W or J/s]
For automobile application,
It is common to see the rated Max Power Output as 47 kW @ 6600 rpm.
What does "47kW @ 6600 rpm" means?
That means if the engine speed for that car reached 6600 rpm, it would produce 47 kJ of work per second.
Familiar with the concept of Force, Work and Power yet ?!?!
Perhaps, a case study is better to address our topic here.
Case Study:
The relationship between vehicle mass and engine size that affect the fuel economy of the vehicle.My Hypothesis:
There must be a balance between the mass and engine size in order to yield a good fuel economy.Test Scenario:
Each car would carry 4 person (included driver) and travel from 30km/h to 50km/h on a flat road along a straight path.
Assumptions and Considerations:
1. Frictional Forces are not considered.2. Combustion efficiency, power transmission efficiency and energy conversion are ignored and assumed to be 100%.
3. Assume the engine power output has linear relationship with the engine speed for all cars in this studies.
4. Most cars achieved acceleration from 30 km/h to 50km/h when the engine speed was around 1750rpm to 2000 rpm. Hence, the median value of the engine speed range (which is 1875 rpm) will be used as the reference engine speed value when the engine is doing work to speed up from 30km/h to 50km/h.
5. Flat road indicated no gravitational (potential energy) effect.
6. Straight path indicated no centripetal force to be considered.
(Note: With all the assumption above, it is an ideal case hence the reality is expected to be worse than this ideal case)
Data:
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| Nine Selected Honda Models Source: http://www.honda.co.jp/ |
Calculation Methodology:
1. How much work done needed from 30km/h to 50km/h?With assumption#5. assumption#6 and derivation from the previous post.
m = dry mass of vehicle + total mass of passengers + mass of driver
(assume average mass for a person is 65 kg)
v = 50km/h = 13.89 m/s
u = 30km/h = 8.33 m/s
Energy Density of Gasoline = 34.2 MJ/L (where L is litre)
With assumption#3,
4. How long does it take from 30km/h to 50km/h with engine power output at 1850 rpm?
Recalled 1 kW = 1 kJ/s
Calculations and Tabulated Data:
With the above data from Honda Official Site, assumptions and the calculation methodology, I present your my finding on the relationship among engine size and the total mass of the vehicle as tabulated below:
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| Calculated data in the case when there is the driver only. |
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| Calculated data in the case when there are 4 person included the driver. |
Data Digestion:
Regardless 1P case or 4P case, Newton is RIGHT.Lighter the car is, more fuel is saved
What is the significance of time taken???
The time taken refer the amount of time needed to accelerate from 30km/h to 50km/h.
Time is a more familiar and common quantity to most readers where they judge the time taken to determine the car's power. No doubt, shorter the time, more powerful the car is.
There are 9 Honda Models but only have 5 categories of engine in term of engine size:
- 0.66 L engine
- 1.3 L engine
- 1.5 L engine (most common)
- 2.0 L engine
- 2.4 L engine
As shown, Honda Civic Type R [FK2] is the most powerful vehicle in my data as it takes 1.4s to speed up from 30km/h to 50km/h whereas Honda NBox G Turbo takes 4.4s to accelerate.
Of course, Honda Civic Type-R is an outlier because it is designed based on racing car specification. With balance in mass and engine size, it is more powerful than Honda Odyssey Absolute although Honda Civic Type-R used 2.0L engine as compared to Honda Odyssey Absolute that used a 2.4L engine.
Similarly, above calculation also show that Honda's K20A engine is the most powerful when comparing other engine at 1850 rpm engine speed.
However the flaw of being the most powerful engine is more fuel consumption.
As compared to Honda Civic Type-R
Other models like,
Honda Fit/Jazz RS takes 1.2s longer but save 0.60ml of gasoline;
Honda Fit/Jazz G13 takes 1.5s longer but save 0.69ml of gasoline;
Honda Verzel/HRV takes 1.5s longer but save 0.36ml of gasoline.
On the other hand, why it is bad to take longer time to achieve 50km/h from 30km/h ???
Lighter car is equipped with smaller engine size since the engine didn't really need to do much work to move the car therefore it is expected to have good fuel economy.
However, Kei Car with 0.66L engine takes more than 3s to achieve that acceleration as shown in the calculated data. In reality, human being are impatient such that we prefer to accelerate in a shorter time.
Let refine our experiment such that we set the acceleration from 30km/h to 50km/h duration to be fixed which is 3s. As a result, vehicle that takes more than 3 secs with 1850 rpm engine speed will do more work per seconds. Similar calculation approach, engine speed for Honda NBox-G Turbo need around 3207 rpm. By considering the work done rate to max power ratio, the kei car need to achieve 53.4% of its MAX POWER OUTPUT. At this stage, the car engine will produce more noise and more potential that lead to engine inefficient. Therefore, can you imagine that you drove Honda NBox in full loading and maintain the speed around 70-80km/h??? In long term, it is very unhealthy to your car engine.
As discussed in previous topics, lighter car saved more fuel.
Hence, Honda S660 has the best fuel economy rating in theory.
With well balance between vehicle mass and engine size,
Honda Civic Type-R uses the most powerful engine such that it take little time to accelerate.
Similarly,
Honda Fit/Jazz G13 has the most optimum performance in both engine power output and fuel economy rating.
The beauty of ideal condition is that:
We know the perfect results.
In reality, conventional gasoline powered vehicle is very inefficient.
Depend which cars model, its overall efficiency is 1-10% only.
For example, Honda Fit/Jazz G13 need 65787J of gasoline to accelerate from 30km/h to 50km/h. If the overall efficiency is 10% only, it need 657870 J of gasoline which is 19.2ml of gasoline instead of 1.92ml.
In addition, the major flaw in the analysis is I excluded the engine torque consideration. In engine structural design, it is hard to improve engine torque and engine power output together.
In the engine, we have cylinders and its volume determine the engine size.
In automotive terminology, they would call them bore and stroke instead of diameter and length respectively as shown in this picture.
In engine design, Bore:Stroke ratio will determine both the engine torque and the engine output.
If the bore is a lot longer than the stroke, the engine is expected to have higher power output.
If the stroke is a lot longer than the bore, the engine is expected to have greater engine torque.
For practical application, we need both optimum value however truck or jeep might prefer higher engine torque. If you have high power but small torque, it is likely that the car need to do double work to produce the torque to move. If you have high torque but low power, it is harder for the car to accelerate.
Of course, I am still learning the relationship between engine torque and the power output and hopefully it will be discussed in the future.
After reading these 3 posts about fuel consumption, I hope my fellow readers have better understanding to choose a car with optimum performance in both engine power output and fuel economy.
Ideally, if I was a big fans of Honda and I have a family of 4, my wife and two working children (a daughter and a son).
I would buy myself
A Honda Fit RS for family vacation (or my own transportation).
A Honda N-Box for my wife to do shopping or my daughter (it is a cute design)
A Honda Fit G13 for either one of my children.
A Honda S660 for me to bring my wife to celebrate our own events without the children.
Of course, Honda Civic Type-R is an outlier because it is designed based on racing car specification. With balance in mass and engine size, it is more powerful than Honda Odyssey Absolute although Honda Civic Type-R used 2.0L engine as compared to Honda Odyssey Absolute that used a 2.4L engine.
Similarly, above calculation also show that Honda's K20A engine is the most powerful when comparing other engine at 1850 rpm engine speed.
However the flaw of being the most powerful engine is more fuel consumption.
As compared to Honda Civic Type-R
Other models like,
Honda Fit/Jazz RS takes 1.2s longer but save 0.60ml of gasoline;
Honda Fit/Jazz G13 takes 1.5s longer but save 0.69ml of gasoline;
Honda Verzel/HRV takes 1.5s longer but save 0.36ml of gasoline.
On the other hand, why it is bad to take longer time to achieve 50km/h from 30km/h ???
Lighter car is equipped with smaller engine size since the engine didn't really need to do much work to move the car therefore it is expected to have good fuel economy.
However, Kei Car with 0.66L engine takes more than 3s to achieve that acceleration as shown in the calculated data. In reality, human being are impatient such that we prefer to accelerate in a shorter time.
Let refine our experiment such that we set the acceleration from 30km/h to 50km/h duration to be fixed which is 3s. As a result, vehicle that takes more than 3 secs with 1850 rpm engine speed will do more work per seconds. Similar calculation approach, engine speed for Honda NBox-G Turbo need around 3207 rpm. By considering the work done rate to max power ratio, the kei car need to achieve 53.4% of its MAX POWER OUTPUT. At this stage, the car engine will produce more noise and more potential that lead to engine inefficient. Therefore, can you imagine that you drove Honda NBox in full loading and maintain the speed around 70-80km/h??? In long term, it is very unhealthy to your car engine.
Conclusions:
Honda Manufacturers will equip the vehicle with suitable engine size by considering the overall loading. Greater mass require greater engine size.As discussed in previous topics, lighter car saved more fuel.
Hence, Honda S660 has the best fuel economy rating in theory.
With well balance between vehicle mass and engine size,
Honda Civic Type-R uses the most powerful engine such that it take little time to accelerate.
Similarly,
Honda Fit/Jazz G13 has the most optimum performance in both engine power output and fuel economy rating.
Further Reminders
Keep remembering that the whole analysis in this post is purely IDEALISTIC.The beauty of ideal condition is that:
We know the perfect results.
In reality, conventional gasoline powered vehicle is very inefficient.
Depend which cars model, its overall efficiency is 1-10% only.
For example, Honda Fit/Jazz G13 need 65787J of gasoline to accelerate from 30km/h to 50km/h. If the overall efficiency is 10% only, it need 657870 J of gasoline which is 19.2ml of gasoline instead of 1.92ml.
In addition, the major flaw in the analysis is I excluded the engine torque consideration. In engine structural design, it is hard to improve engine torque and engine power output together.
In the engine, we have cylinders and its volume determine the engine size.
In automotive terminology, they would call them bore and stroke instead of diameter and length respectively as shown in this picture.
If the bore is a lot longer than the stroke, the engine is expected to have higher power output.
If the stroke is a lot longer than the bore, the engine is expected to have greater engine torque.
For practical application, we need both optimum value however truck or jeep might prefer higher engine torque. If you have high power but small torque, it is likely that the car need to do double work to produce the torque to move. If you have high torque but low power, it is harder for the car to accelerate.
Of course, I am still learning the relationship between engine torque and the power output and hopefully it will be discussed in the future.
After reading these 3 posts about fuel consumption, I hope my fellow readers have better understanding to choose a car with optimum performance in both engine power output and fuel economy.
Ideally, if I was a big fans of Honda and I have a family of 4, my wife and two working children (a daughter and a son).
I would buy myself
A Honda Fit RS for family vacation (or my own transportation).
A Honda N-Box for my wife to do shopping or my daughter (it is a cute design)
A Honda Fit G13 for either one of my children.
A Honda S660 for me to bring my wife to celebrate our own events without the children.
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