10 Attachment(s)
Aerodynamics of the new Mirage (0.27-0.31 drag coefficient varies with options)
http://mirageforum.com/forum/attachm...1&d=1385312170
The Mirage / Space Star aerodynamic design is better than average for a small 4-door hatchback body. Mitsubishi lists drag coefficients (Cd) ranging from 0.27 to 0.31, depending on drivetrain and options, with the Japanese and Euro market trims having the lowest Cd:
That Cd contributes to the Mirage being the most fuel-efficient gasoline (non-hybrid) registered car in Japan.
Mitsu U.K. says:
Quote:
The most advanced research has gone into the aerodynamics of the Mirage resulting in one the most aerodynamic vehicles in its class. The overall shape has been optimised to fine tune its
drag coefficiency to just 0.27 and the sleek body, clam-shell bonnet and tapered roofline create the striking profile of a car engineered from the ground up for fuel economy and CO
2 efficiency. (
Source)
Notable aero design features (varies by model/options):
Of course, reducing aero drag isn't just about improving fuel economy. It also results in a quieter cabin (less wind & engine noise) and more engine power in reserve at highway speeds (faster high speed acceleration).
See also: Estimating Mirage gas mileage: May be highest MPG in its class in U.S./Canada
Mitsu provided technical drawings in some of its brochures which we can use to derive frontal area, and ultimately CdA. That figure is more useful for comparing the car's total drag area to values known for other cars.
Could the Mirage's drag coefficient be made even better?
Yes, at a cost (either financial, or affecting driveability). Here are a few drag-reducing modifications that already exist on other production vehicles that could be adapted to the Mirage:
- variable automated grille blocking
- reduced ride height
- additional underbody panels
- a larger "Kammback" drag reducing spoiler
Don't aerodynamics only matter at highway speeds?
People often incorrectly assume that aerodynamic design elements "only matter above 60 MPH" (or some other arbitrary high speed). The fact is, many vehicles burn more than half their fuel to overcome air drag at speeds as low as 25-30 mph (40-50 km/h), including the Mirage at Cd = 0.30.
So, yes, aero is critically important to highway fuel economy. But it also affects city numbers.
Of course the effect is exponentially greater the faster you go. When you double your speed, you cube (x8) the power needed to overcome aero drag.
EDIT: See post #13, below, for calculations that show a difference of 5 MPG between a Mirage with a Cd of 0.27 vs. a Mirage with a Cd of 0.31 cruising a speed of just 50 mph (80 km/h).
1 Attachment(s)
Estimating frontal area and total drag (CdA)
http://mirageforum.com/forum/attachm...9&d=1358580581
Based on dimensional drawings of the car from an Australian brochure, I've calculated the projected frontal area (A) of the car to be 2.04 square meters (21.97 square ft.). (Later I'll post how I got these numbers.)
- That means the total drag area (Cd * A) of the lowest drag version of the car (Cd = 0.27) is:
0.551 sq. m (5.93 sq. ft)
- For the highest drag configuration (Cd 0.30), total drag area is:
0.612 sq. m (6.59 sq. ft)
Compare to other small cars:
http://ecomodder.com/wiki/index.php/...t_of_Drag_List
1 Attachment(s)
How I derived frontal Area (A)
This technical drawing was provided in the Mirage brochure (PDF) from Australia:
http://mirageforum.com/forum/attachm...0&d=1358620150
It's a decent candidate image to use because it doesn't have perspective distortion we'd typically get in a front-on photograph. Also, Mitsu conveniently provided exact dimensions in millimeters, so we have an accurate reference to figure out the pixels:mm ratio in the image.
- First, I enlarged the image to make it easier to work with.
- Next, I traced the car's outline with the selection tool, then adjusted the selection so it was centered on the line.
- Then I bucket-filled the selected area with black, and deleted everything outside the selected area
- I converted the image to black and white (2 colours only) and was left with:
http://mirageforum.com/forum/attachm...9&d=1358580581 - Photoshop has a handy Histogram tool (Image > Histogram) which will tell you the number of pixels for each individual colour in the image
- Since it's a 2-colour image only, we can easily see that the black pixels represent only the car's frontal area (153501 pixels, in the image I was working with --- a larger one than the attachment image in this post).
- From here on, it's just math:
(A) the diagram dimensions showed the width of the car as 1665 mm.
(B) Using a measurement tool, I found the pixel width of the car in my working image was 457 px.
(C) So we now have a ratio of 1665 mm/457 px, or 3.643 mm/px.
(D) But a pixel is a square, so we have to sqare the number of mm to get 13.274 square mm/px.
(E) Last step: how many pixels in the car? 153747 px * 13.274 mm2/px = 2040811 mm2, or 2.041 square meters.
QED!