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July 23, 2013
Diesel Explorer: Choosing a chassis
Having settled in on the Cummins B3.3 for an engine, the next step was to choose a vehicle to put it in. I had some rough requirements in mind:
- Seating capacity: At least 4 people
- Air conditioning and decent acoustics/sound system
- 4-wheel drive
Since the original inspiration for the project was a Cummins B3.3 in a Jeep Wrangler (YJ), that was an obvious candidate, but I also had an affinity for and experience with Ford body styles and interiors. The decision came down to two potential chassis: Wrangler vs. Explorer. Both satisfied the basic requirement above, but there were some more criteria to consider—fitment, noise and comfort, emissions, initial cost, and fuel economy.
Fitment
An obvious requirement was that the engine fit inside the vehicle’s engine bay. This contest goes to the Wrangler. The engine bay is huge, and the hood opens very wide. With the Explorer, a body lift would most likely be necessary.
Winner: Wrangler
Noise/acoustics and overall comfort
The fun and excitement of the Jeep’s removable top comes along with a noisy ride, and poor acoustics. For me, the Explorer’s acoustics and comfort were worth the somewhat less hip styling. Electric windows and locks were icing on the cake.
Winner: Explorer
Initial cost
On the used market, Wranglers fetch about 2–3 times the amount Explorers do. Spending less on a chassis would save more for the engine, giving the Explorer the edge.
Winner: Explorer
Emissions
Since this vehicle would likely throw many error codes with an OBD-II computer, it was important to find a vehicle that was OBD-I. The TJ body style is not available in the OBD-I years, while the 2nd generation Explorer body style was just introduced in 1995, barely making the cutoff. The Explorer body style and interior both beat the YJ, as discussed above, so the Explorer gets the nod here.
Winner: Explorer
Fuel economy
The most important metric—and the main goal of the project—was to achieve high fuel efficiency. Thus, this metric would likely decide the contest. Properties for the two vehicles are tabulated below. On one hand, the Wrangler is lighter than the Explorer, requiring less energy to get up to speed. However, the Wrangler is also less aerodynamic, consuming more energy while cruising. To compare the two, a typical driving scheme was devised, for which rough estimates for fuel consumption were calculated. This driving scheme included
- Accelerating 0–40 mph in 10 seconds
- Cruising at 40 mph for 5 miles
Explorer Wrangler Mass (kg) 1806 [1] 1331 [2] Drag coefficient 0.43 [3] 0.55 [3] Frontal area ($\text{m}^2 $) 2.3 2.3 Calculating fuel consumption during cruising
This was the easy part. Neglecting friction in the tires and drivetrain—which would be more or less equivalent between both vehicles—energy consumption during cruising can be estimated by integrating the drag force over the distance traveled.
Drag force is estimated using the formula \(F_D = \frac{1}{2}\rho A_f C_D V^2,\)
where $ \rho $ is the density of air, $ A_f $ is the vehicle’s frontal area (approximately equal for both Explorer and Wrangler), $ C_D $ is the vehicle’s drag coefficient, and $ V $ is the cruising speed. Since speed is constant, the integration simplifies to multiplication, giving (in Nm)
\[E_{\mathrm{cruise}} = 3,550,673 C_D.\]Calculating fuel consumption during acceleration
Calculating the energy required during acceleration included consideration of both drag and inertial forces. Since drag is not constant during acceleration, we will use a more general method and integrate the varying power over the acceleration time, which is simply the force multiplied by velocity, or
\[E_{\mathrm{acc}} = \int_0^{10} \left( F_D V + maV \right) \, \mathrm{d}t.\]Integrating the constant acceleration gives the vehicle speed as a function of time, i.e.,
\[V(t) = \frac{\mathrm{d}x}{\mathrm{d}t} = 1.79 t.\]Boiling this all down and substituting all but the variables that differ between the vehicles, we obtain
\[E_{\mathrm{acc}} = 19,787 C_D + 160.2m.\]The table below summarizes the results for both vehicles. As you can see, these estimates show the Explorer consuming 16% less energy than the Wrangler, making it a better choice with regards to fuel economy.
Explorer Wrangler Energy consumed during cruising (Nm) $ 1.53 \times 10^6 $ $ 1.95 \times 10^6 $ Energy consumed during acceleration (Nm) $ 2.98 \times 10^5 $ $ 2.24 \times 10^5 $ Total energy consumed (Nm) $ 1.83 \times 10^6 $ $ 2.17 \times 10^6 $ Winner: Explorer
Conclusions
Obviously, since this post is in a series about a diesel Explorer, you can guess which chassis won out in the end. The Wrangler is a cool design for sure, but the Explorer’s acoustics, comfort, and most importantly aerodynamics made it my choice.
References
[1] https://www.edmunds.com/ford/explorer/1995/features-specs.html
[2] https://www.edmunds.com/jeep/wrangler/1995/features-specs.html
[3] https://ecomodder.com/wiki/index.php/Vehicle_Coefficient_of_Drag_List
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June 23, 2013
Diesel Explorer: Introduction
I first became interested in diesel engines after learning about the diesel cycle in a thermodynamics class. This led to my first diesel engine swap—a 3.9 L Cummins 4BT into a heavily modified Ford Ranger. I boxed the frame, installed 1-ton axles, air lockers; everything to make a really great off-road machine that still managed an MPG somewhere in the 20s. It was cool, but not exactly what I wanted since it was a bit impractical, and the stiff frame made the truck vibrate too much on the highway. I took it all apart, sold the valuable parts, and scrapped the rest. My diesel project was gone, but my interest in diesels was not. I decided I wanted a practical diesel SUV that got good mileage—nothing spectacular, just a good solid vehicle. This time, I would take what I had learned from that and my previous projects, but think long and hard about the goals of the project before starting.
At this point in time—around 2007–2008—I believe the only commercially available (in the US) diesel SUVs were the Jeep Liberty and the way-out-of-my-price-range Volkswagen Toureg, neither of which got the mileage I was looking for. I could have went for a Volkswagen TDI Golf, but I really like having 4 wheel drive for snow and mildly muddy roads/trails.
Being a Cummins enthusiast, I used to visit the forums at dieseltruckresource.com, where I stumbled upon an inspiring thread by forum member TDIwyse. He had acquired a 3.3 L Cummins turbo diesel engine and put it in a Jeep YJ. The swap alone looked really clean and done quite well, but the part I was most interested in was the mileage. He claimed he was getting MPGs in the 30s. I decided my project would be built around that engine—a Cummins B3.3.