XL1 dive and drive: Volkswagen aggressively optimizes for efficiency in its sleek diesel plug-in hybrid
24 June 2013
|XL1s ready to roll. Click to enlarge.|
During a media event in Wolfsburg, Germany, Volkswagen provided a medium dive into details of its two-seater XL1 Super Efficient Vehicle diesel plug-in hybrid (earlier post) as well as some driving time on local roads and highways.
The XL1 is aggressively optimized for efficiency in all areas of its design and technology—from materials (carbon fiber reinforced polymer monocoque); to powertrain (0.8L two-cylinder diesel, 20 kW electric motor and 5.5 kWh Li-ion pack with efficiency-optimized control and brake regeneration strategies); to lightweighting (little baffling to shield the driver from engine, chassis and road noises); to aerodynamics (with a Cd of 0.189, the XL1 bests the only other production vehicle that has come close, the GM EV1’s 0.19; the original Honda Insight hybrid had a Cd of 0.25). It reflects, the Volkswagen team members said, an effort by the company to deliver the most efficient vehicle they could—but one that could still be series-produced, homologated and sold—with current technologies.
|Rear view, with engine compartment with air intake labeled. Click to enlarge.|
The lightweight XL1 (795 kg, 1,753 lbs) delivers NEDC fuel consumption of 261 mpg (0.9 l/100 km) with CO2 emissions of 21 g/km and an all-electric range of 31 miles (50 km) as well as a total range of 310 miles (499 km). The XL1 has a top speed (electronically controlled) of 160 km/h (99 mph); cruising at a constant 100 km/h the XL1 uses only 8.3 hp (6.2 kW). In all-electric mode, the XL1 requires less than 0.1 kWh to cover more than 0.6 miles (one kilometer), based on the NEDC.
0.1 kWh per km equates to 0.161 kWh/mile, or 16 kWh/100 miles. Even accounting for differences resulting from variations in drive cycles, XL1 appears to deliver significantly better electric fuel consumption than the EPA-rated electric or plug-in vehicles on the US market.
|Comparison of electric drive fuel consumption (kWh/100 miles). Data: Volkswagen, US EPA and DOE. Click to enlarge.|
Volkswagen has begun limited production of the XL1; 50 units are being built initially, with 250 slated for build.
Initial visual impressions. XL1 is visually striking; 153.1 inches long, 65.6 inches wide, and only 45.4 inches tall. By comparison, a Volkswagen Polo is slightly longer (156.3 in) and wider (66.2 in), but is significantly taller (57.6 in). Even a sportscar like the Porsche Boxster is 5.1 inches taller than XL1. (When you scoot alongside a heavy-duty truck and trailer combo, you realize how low to the ground you are.) Its aerodynamic shape appears bio-inspired by aerodynamic swimmers—similar to a dolphin, as Volkswagen says, or to a boxfish. (Earlier post.)
|The XL1 with door-mounted camera clearly visible. Click to enlarge.|
XL1 represents the third evolutionary stage of Volkswagen’s 1-liter car strategy, referring to the goal set by Prof. Dr. Ferdinand Piëch—currently Chairman of the Supervisory Board of Volkswagen AG—more than 10 years ago to produce a practical car that had a combined fuel consumption of one liter per 100 km (235 mpg).
With XL1, the engineers and designers successfully came up with a body design that delivers more everyday utility than the two previous prototypes. In the L1, the 1-liter car that was shown in 2002 and 2009, the driver and passenger sat behind each other for optimal aerodynamics; in the XL1, the two occupants sit slightly offset, side by side, almost as in a conventional vehicle.
The XL1 uses scissor doors; hinged low on the A-pillars and just above the windshield in the roof frame, they swivel upwards and slightly forwards as well. The doors also extend far into the roof. When they are opened, they create an large amount of entry and exit space.
|The offset seating in the XL1 not only enables shoulder room for the driver and passenger, but also provides the location for the placement of the 5.5 kWh Li-ion battery pack—which is sited in front of the passenger area. Click to enlarge.|
In profile, the roofline traces an arc from the A-pillar back to the rear. The rear wheels are fully covered to prevent turbulence, while airflow around the wheel arches is optimized by small spoilers in front of and behind the wheels. There are no door mirrors: these are replaced by small cameras known as e-mirrors (digital outside mirrors) that send images to two displays, one in each door.
Despite its aggressive design, the XL1 can still be considered a practical car. It is roomy and pleasant for two passengers; entry is convenient—despite the low body—due to the large door openings; it offers an adjustable driver’s seat and steering wheel; comes with the expected full range of multimedia devices with radio, navigation, Internet; offers air conditioning and PTC auxiliary heating; and has a cargo capacity of approximately 4.2 cubic feet. Cruise control, ABS and electronic stabilization program, and a parking assistant all contribute to a “modern” level of driver assistance.
Driving—initial impressions. The XL1 is not a sprinter (0-100 km/h in 12.7 seconds), but it doesn’t feel as though it lacks for power or struggles at its higher speed range. Attaining cruise speed or overtaking on the highway just takes a bit longer. XL1 has no power steering, but is very nimble and easy to control due to its aerodynamic shape and light weight.
Coasting at higher speeds is noticeably different than in other vehicles—with its very low coefficient of drag, the XL1 feels as if it is slipping through the air, not battering its way ahead.
The brake regeneration strategy is very aggressive. Unlike the e-Golf, for example, which uses its paddle shifters to allow the driver to click through three different settings for regen (earlier post), XL1 goes for as much energy as it can get, every time.
It is relatively easy to become accustomed to the position of the e-mirror screens in the doors and the lack of outside mirrors. It’s a bit more difficult not to have a central rear-view. Since the engine compartment is behind the driver and passenger’s head, there is no rear window, but a central rear-view camera display would be helpful, especially with backing up.
Occupants of the XL1 cabin will experience more noise than the level to which they are likely accustomed. Sound baffling brings extra weight, and the VW engineering team was determined the break the upward spiral of weight gains in vehicles (e.g., more features makes a heavier car, which requires more power, which requires a larger engine, etc.). As a result, there is minimal shielding of the noise that accompanies any car when it is driving.
The two-cylinder diesel is very audible when it runs. Braking too yields its own sound with which drivers will be largely unfamiliar. The ceramic brakes make a sound similar to sliding out on gravel, or swooshing through water. Until the driver becomes accustomed to the noise, in other words, it sounds as if something might be amiss. Nor is there much cushioning from road noise. Rolling over a manhole cover, for example, is initially a bit startling.
However, those design decisions contributed to the extreme fuel efficiency of the XL1, the engineers point out.
Powertrain. The XL1 is a rear wheel-drive plug-in parallel hybrid, using a 48 hp 0.83L, two-cylinder TDI common-rail turbodiesel with aluminum block; a weight-optimized magnesium seven-speed DSG dual-clutch automatic transmission; a 27 hp traction motor; and a water-cooled 5.5 kWh Li-ion battery pack. The entire hybrid unit is housed between the driven rear wheels. The hybrid module, which incorporates the electric motor and the clutch, is positioned between the TDI engine and the seven-speed DSG transmission.
|The hybrid power unit. Click to enlarge.|
|Layout of the XL1 powertrain. Note the position of the battery pack on the passenger side, which also helps with weight distribution in the car (with one driver). Click to enlarge.|
The XL1 is not a product of any of Volkswagen’s modular Baukästen (assembly kits such as the MQB, MLB or MSB, earlier post). Nonetheless, some of the learnings from the XL1 will flow back to inform the evolution of the different kits, such as to the MQB which has its own plug-in hybrid powertrain, to make its debut in the A3.
|Components of the XL1 plug-in power unit. Click to enlarge.||Components of MQB plug-in hybrid power unit. Click to enlarge.|
The 5.5 kWh lithium-ion battery—integrated in the front section of the XL1—supplies the electric motor with 220 volts of electrical energy. The battery comprises 60 25Ah cells, with a carbon-fiber case. The power electronics system manages the flow of high voltage energy to and from the battery or electric motor and converts direct current to alternating current. The XL1’s 12-volt electrical system is supplied via a DC/DC converter and a small auxiliary battery.
The TDI delivers 89 lb-ft (120 N·m) of torque from its 830 cc. The electric motor, which generates up to 103 lb-ft (140 N·m) of torque from a standstill, works as a booster to support the TDI engine when full system power is required. Together, the TDI engine and electric motor deliver maximum torque of 103 lb-ft along with 68 hp in boosting mode.
|Comparing the rear-wheel drive XL1 plug-in hybrid powertrain layout to that of the front-wheel drive MQB plug-in hybrid powertrain layout. Click to enlarge.|
As well as boosting the TDI engine under hard acceleration, the electric motor can also power the XL1 on its own for a distance of up to 31 miles. In this mode, the TDI engine is decoupled from the drivetrain by disengaging a clutch, and is shut down.
Meanwhile, the clutch on the gearbox side remains closed, so the DSG is fully engaged with the electric motor. The driver can choose to drive the XL1 in pure electric mode, provided that the battery is sufficiently charged, by pressing a button on the instrument panel.
Restarting the TDI engine is a smooth process. While driving, the electric motor’s rotor is sped up and is very quickly coupled to the clutch in a process known as “pulse starting”. This accelerates the diesel engine to the required speed and starts it, so the driver hardly notices the transition. In certain operating conditions, the load of the TDI engine can be shifted so that it operates at its most favorable efficiency level.
The XLI can be also be put into a charging mode, which runs the engine at a slightly higher point that required, using the excess power to generate electricity to recharge the battery.
|Power and torque curves for the XL1. Click to enlarge.|
(In our brief driving experience, whenever the TDI kicked in, it went straight to 2,000 rpm (according to the tachometer). Volkswagen engineers said that seemed a bit high.)
Some of the parameters used to realize the optimum propulsion mode for the given conditions are accelerator pedal position and demanded engine load, as well as the energy supply and mix of kinetic and electrical energy at any given time. The already complex control strategy is made even more complex by the requirements of the diesel engine, as well as emissions control (e.g., DPF regen events, etc.)
The gears in the DSG transmission are also always selected with the aim of minimizing energy usage.
The 0.8-liter two-cylinder TDI engine was derived from a 1.6-liter four-cylinder engine, so it shares the 88-mm bore spacing and a bore and stroke of 81.0 mm x 80.5 mm. The XL1’s engine also shares key internal modifications for reducing emissions, which include specially formed piston recesses for multiple injection and individual orientation of the injection jets. The common-rail diesel’s smooth running properties were transferred to the two-cylinder engine, aided by a balancer shaft that is driven by and turns at the same speed as the crankshaft.
The TDI’s aluminum crankcase is constructed with extremely low tolerances, which in turn leads to very low frictional losses. Exhaust gas recirculation, an oxidation catalytic converter, and a diesel particulate filter are used in order to minimize emissions: the XL1’s engine is already compliant with the Euro-6 emissions standard.
The vehicle’s cooling system is also designed to maximize efficiency. The mechanical water pump actively cools the engine only when operating conditions require it. The system includes automatically controlled air intakes at the front of the vehicle to reduce drag. A second electric water pump, used only when needed, circulates a separate lower temperature loop to cool the starter generator and the power electronics. This thermal management strategy also contributes towards reduced fuel consumption.
Materials and weight. The XL1 utilizes lightweight and extremely strong carbon fiber reinforced polymer (CFRP) construction for the monocoque, all exterior body parts, and components such as the anti-roll bars. Produced using the Resin Transfer Molding process (RTM), the density/specific gravity of the CFRP is around 20% of a comparable steel exterior structure’s. The CFRP parts are as strong as comparable steel or aluminum parts, yet the exterior skin of the XL1 is just 0.05 inches thick.
|CFRP monocoque. Click to enlarge.||Assembly of front structure. Click to enlarge.|
|Assembly of rear structure. Click to enlarge.||Manufacturing and installing swivel door (CFRP – Prepreg). Click to enlarge.|
Compared to manufacturing CFRP in a pre-preg process, the RTM process is more economical—with lower costs at higher volumes—because it can be automated. The RTM parts are produced in multi-shell heated and vacuum-sealed molds. This involves injecting liquid resin at high pressure into the tool which contains the semi-finished carbon material. The part is then cured in the mold.
The XL1 weighs just 1753 pounds (795 kg), consisting of:
- 501 lb (227 kg) for the drive unit including battery (231 lb, 105 kg);
- 337 lb (153 kg) for the running gear;
- 176 lb (80 kg) for the interior;
- 232 lb (105 kg) for the electrical system; and
- 507 lb (230 kg) for the body, including crash structure and windshield.
A total of 21.3% of the new XL1, or 373 lb (169 kg), consists of CFRP. In addition, Volkswagen uses lightweight metals for 22.5% of all the parts (395 lb, (179 kg), with just 23.2% (406 lb, 184 kg) of the car being constructed from steel and iron.
As an example, the seats are made from CFRP and weigh 25.6 lb (11.6 kg) each, or about half the mass of a normal car seat. The seats are also very comfortable.
As another example of the use of lightweight materials, polycarbonate side windows weigh about a third less than conventional windows: a pane of glass on XL1 would weigh 16.5 lb (7.5 kg), whereas the polycarbonate ones weigh 11.5 lb (5.2 kg) each.
(Volkswagen purchased an 8.18% stake in SGL Group—one of the world’s leading manufacturers of carbon-based products with a portfolio ranging from carbon and graphite products to carbon fibers and composites. (Earlier post.) SGL is partnering with BMW in a joint venture to produce carbon fiber for automotive applications; BMW owns 15.6% of SGL.)
The weight of the running gear is also reduced by the use of many lightweight components. Aluminum is used for parts such as the suspension arms, brake calipers, dampers, and steering gear housing; the anti-roll bars are made from CFRP; the brake discs are ceramic; the wheels are case in magnesium alloy; and the steering wheel body is plastic.
The car is also designed to reducing rolling resistance, with friction-optimized wheel bearings and driveshafts, as well as ultra-low rolling resistance Michelin tires, sized 115/80 up front and 145/55 R16 at the back.
Safety. The CFRP monocoque is not only light weight, but provides a survival cell for the driver and passenger. This is achieved by the intelligent design of load paths, including the use of sandwich structures in the monocoque, while the front and rear aluminum crush structures absorb a large share of energy in frontal and rear collisions.
Frontal crash protection measures include aluminum crash tubes with crossmembers, a sandwich structure (CFRP / PMI foam) in the firewall, and an airbag in the steering wheel.
The CFRP doors have aluminum impact beams to absorb crash energy and the door frame also minimizes intrusions into the safety cell. A great deal of attention was also paid to extricating occupants in the event of a rollover collision: pyrotechnic separating screws are used to simplify opening of the doors, which ordinarily open upwards.
Rear crash protection measures also include aluminum crash tubes with cross members.
|Click to enlarge.|
Volkswagen hosted Green Car Congress at the Wolfsburg event.
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