The Ward’s 10 Best Engines competition has recognized outstanding powertrain development for 17 years. In this installment of the 2011 Behind the 10 Best Engines series, Ward’s looks at the development of the BMW 1.6L turbocharged DOHC DI I-4 for the Mini brand.
We toss around the term “trend-setting” in the auto industry when referring to innovative styling or features or even a new market segment.
But probably no vehicle in automotive history has earned the modifier more than the tiny, transverse-engine, front-wheel-drive British Motor Mini launched in 1959 and produced (almost unbelievably) until 2000.
Prior to the Mini, nearly all cars were driven by their rear wheels, and the few FWD exceptions had their engines mounted front-to-rear (north-south) with much of their mass ahead of the front wheels.
Designed by a brilliant engineer named Alec Issigonis (later knighted for this and other feats), the Mini’s trend-setting breakthrough was mounting its I-4 engine transversely (east-west) under the hood.
This orientation substantially reduced its front overhang, shortened the engine compartment and thereby improved weight distribution and allowed 80% of its floorpan length to accommodate people and stuff.
Today, for both packaging and fuel efficiency, most vehicles are FWD and have transversely mounted engines. The iconic, British-built Mini, now owned, designed and marketed by BMW, has been back since its 2002 relaunch proving every day that Americans are willing to pay good money for small cars that are not just fuel-efficient but also fun to drive.
The original ’02 Mini Cooper was powered by a 1.6L engine built in Brazil by Tritec Motors, a BMW/Chrysler joint-venture that has been dissolved.
The 4-cyl. was good for 115 hp, 110 lb.-ft. (149 Nm) of torque and 25/32 mpg (9.4-7.4 L/100 km) city/ highway in the base manual-transmission coupe. The supercharged version in Cooper S models upped the output to 163 horses and 155 lb.-ft. (210 Nm) and 25/34 mpg (9.4-6.9 L/100 km).
The ’06 model year brought a new all-aluminum 1.6L “Prince” I-4, jointly developed with PSA Peugeot Citroen, built in Hams Hall, U.K. and featuring BMW’s Valvetronic variable-valve-lift technology.
It bumped base power to 118 hp and torque to 114 lb.-ft. (155 Nm) with 26/34 mpg (9-6.9 L/100 km) ratings, while its turbocharged (not supercharged) direct-injected variant (without Valvetronic) pumped out a healthy 172 ponies and 177 lb.-ft. (240 Nm), with a temporary overboost feature good for 192 lb.-ft. (260 Nm). Fuel economy for the manual version was 25/32 mpg (9.4-7.4 L/100 km).
For ’11, BMW upped output again by adding direct injection and double VANOS (variable valve timing) to the naturally-aspirated 4-cyl., plus Valvetronic and a new twin-scroll turbocharger to the S version.
That gives base-engine drivers 121 hp and 114 lb.-ft. (155 Nm) of torque and moves S owners up to 181 hp and 177 lb. ft. (240 Nm), with the same 192 lb.-ft. (260-Nm) overboost feature. Manual-transmission coupe ratings now are 29/37 mpg (8.1-6.4 L/100 km) and 27/35 mpg (8.7-6.7 L/100 km), respectively.
The latest twin-scroll Cooper S 4-cyl. turbo earns BMW a second 2011 Ward’s 10 Best Engines Award (along with BMW’s 3.0L N55 turbocharged I-6). After averaging 34/35 mpg (6.9-6.7 L/100 km) in an ’11 Cooper S, “while beating it hard,” Ward’s editors exclaimed that it combines “stunning performance and superb fuel economy.”
Mini USA Product Planning Manager Vinnie Kung confirms that soon after introducing the first-generation Mini, BMW was looking to co-develop a new engine with a new partner, which turned out to be PSA.
“Peugeot had a price point they had to meet for their version, so it would not be as contented as ours,” Kung says. “The biggest thing for us was to use BMW technology primarily in the cylinder head, so our head is a dedicated design with our patented Valvetronic technology.”
Kung credits PSA with the bulk of the bottom-end design work. “They wanted to make sure it would fit in many platforms, where we only had one, so the bottom half was prioritized to fit their vehicles. But everything else on our version, including the fuel-injection system and the accessory drive engineering, was done primarily by the BMW Group.”
Kung says the key design objectives were fuel efficiency, manufacturing efficiency, reliability and light weight.
“With any front-drive car, when you take weight off the front, the weight distribution instantly improves. Our ’11 Cooper S hardtop is 61% front, 39% rear, where previously (with the iron-block Tritec engine) it was about 63/37.”
The competition (especially Volkswagen) was fast improving, so performance was critically important. BMW chose to stay with the 1.6L displacement, but strove for the highest specific output for that size. The Cooper S makes 181 hp (113 hp/L) and the hotter John Cooper Works version cranks out 208 hp, or 130 hp/L – better than the Corvette ZR1.
The toughest challenge was packaging. “The PSA cars have more front overhang, but our design team had a very clear short-front-overhang directive,” Kung says.
|Vehicle type||Mini Cooper S|
|Engine||1.6L TurboCharged DOHC I-4|
|Bore x stroke (mm)||77 x 85.8|
|Horsepower (SAE net)||181 @ 5,500 rpm|
|Torque||177 lb.-ft (240 Nm) @ 1,600-5,000 rpm|
|Specific output||113 hp/L|
|Assembly site||Hams Hall, U.K.|
|Application tested||’11 Mini Cooper S|
|Fuel economy EPA city/highway (mpg)||27/36|
The main issue with transverse engines is packaging the front accessory drive. Some engines have accessories hanging all over them, which determines how far forward they hang off the front axle centerline.
“We needed adequate clearance for the cooling system and the head, with its intake on the back and exhaust facing front, the turbocharger in a high location and the primary catalytic converter on its outlet for cold-start emissions. Everything is high and tight,” Kung says.
Especially challenging was effective cooling with different-size radiators for different global applications.
“We have a high climate requirement in the U.S.,” Kung says. “Every car has to be able to operate in very cold and very hot conditions. Any car we sell in New York has to work as well in Texas.
“We had to make sure that the engine would accommodate this with a U.S.-specific radiator. Every quarter inch makes a drastic difference in the way you engineer an engine, because the catalytic converter is right up front and needs adequate space around it for cooling.”
For improved noise, vibration and harshness characteristics, all accessories are mounted directly to the block and cylinder head.
“Bolting the accessories directly to the block and keeping them tight allowed us to package the engine effectively with room for the turbocharger,” Kung says.
“And we use an on-demand water pump with a pulley that's completely wrapped in rubber. The crank pulley uses the backside of the belt to drive the water pump belt through an idler pulley that is engaged and disengaged directly.”
Room for future improvement?
“Everyone has heard that we’re looking at different cylinder counts and different technologies,” Kung says. “It will be a process of optimization of everything we already have. If we can take Valvetronic to the next level, and lubrication systems and accessory drives, and electrify things such as power steering.
“I think in the next three years, when we introduce the next-generation Mini, you will see those technologies. And I would not be surprised to see another 20% improvement in fuel economy.”