Again - my understanding, flawed as it may be, is that Ducati themselves termed the engine an “L” to distinguish it from V configurations, because of their association with Harley type bikes.
From Motomatters:
The L4 - Bad Packaging And Poor Weight Transfer
Ask anyone with even a passing interest in motorcycling what engine a Ducati uses and they will tell you without hesitation that it is a 90° V twin, also called an L twin, because the right angle between the two cylinders makes the configuration look like the letter L. The Bologna factory has been building engines in this configuration for 40 years now, since switching from smaller capacity singles to V twins at the start of the 1970s.
So when Ducati decided to enter MotoGP, they naturally attempted to retain the iconic engine design which has been a key selling point for so many years. Filippo Preziosi was quick to understand that a twin would never be able to produce the horsepower needed to compete in the series, and so concentrated instead on building a four-cylinder engine, built to resemble the 90° L twin as closely as possible. Almost as an act of penance for the extra set of cylinders, the initial plan was for the Desmosedici to use a "twin pulse" or big bang firing order, with the cylinders in each bank of cylinders firing simultaneously, to make the bike sound (and behave) more like a twin. Problems handling the power such a configuration produced meant that Ducati had to switch to a "four pulse" or screamer firing order, each cylinder firing separately, but since then, the factory has oscillated between the two firing orders.
The major benefit of a 90° angle between the cylinders is primary balance, where the motion of each piston in the V is balanced against the other piston. As one piston reaches top dead center, the other is in the middle of its stroke, maintaining its momentum and damping the change in kinetic energy as the first piston switches from upwards to downwards motion. The mechanical balance of the L configuration means that it does not require a large balance shaft to damp the vibrations of the engine, saving power. Balance shafts cost power to drive.
But the biggest problem of the L4 configuration is its size and layout. In the modern era of MotoGP, much of the focus has been on keeping the mass of the bike as centralized as possible. The benefit of centralizing mass is that changing the setup of the bike - its weight distribution, rider position, suspension changes - can be more refined and more predictable. Knowing where all of the mass is makes it easier to calculate how to move it around to achieve the desired effect at a particular track.
The main thrust of mass centralization has been in engine layouts: Suzuki's GSV-R uses a 65° V4 engine, Honda's RC212V uses a 72° V4, and Yamaha's M1 uses an inline 4 to make the engine even more compact, sacrificing a little bit of width for more centralization of mass. Behind the crankshaft, gearboxes are stacked, the rows of gears transferring power from the crankshaft to the rear wheel folded up into a V to shorten the length of the gearbox and keep the mass even closer to the engine's overall center of mass.
The compact engine layouts have a secondary benefit as well: with the engine taking up less space, fuel tanks have migrated to be located underneath the rider's seat, placing that mass (which disappears as the fuel is burned off during the race) close to the center as well. A compact engine gives designers the freedom to place other heavy objects - including the rider - in a range of locations around the bike, to help them achieve whatever goals they may have set themselves.
Nicky Hayden's GP11.1 Ducati Desmosedicis at Laguna SecaAnd here's where Ducati's L4 falls down. The angle between the two cylinder banks makes the engine much longer than its rivals, leaving a large space between the cylinder banks which is filled only with the throttle bodies and airbox/subframe. While the front cylinder bank protrudes through a cutout in the radiators to almost touch the front wheel, the rear cylinder bank slopes back and sits right where Yamaha has its fuel tank.
The 90° angle between the cylinders forces the front cylinders to be angled forward much more than the narrower angle Honda. Visual estimates (the exact data involved is highly sensitive and impossible to obtain) suggest that the front cylinder bank of the Ducati is at 70° from the vertical, while Honda's RC212V is at just 45°. This means that the Honda engine can be moved much further forward and closer to the front wheel than the Ducati, allowing the Honda's chassis designers more freedom in placing the engine. The 18° difference in V angle between the Ducati and the Honda also equates to cylinder banks that are roughly 15% further apart, making the engine correspondingly longer. Move the Desmosedici engine further forward, and you foul the front wheel; move it further back and you drastically shorten the swingarm.
The physical size of the Desmosedici engine - or rather, its rather rangy layout - means that much of the mass of the bike is further back than its rivals, with less freedom for changing weight distribution, especially at the design stage. This difficulty in moving weight distribution is one of the prime candidates for the difficulty the riders have for getting the Desmosedici's front tire up to temperature. While the bike may feel fine and the weight distribution look good on paper, the way the weight transfers under braking and acceleration is different, and this could be what is preventing the riders from getting heat into the tire.
There are several major clues that this is exactly what the problem is with the Desmosedici. Throughout their struggles with the Ducati, Rossi and his vastly experienced - and multiple world championship winning - crew have experimented with some fairly drastic changes to the weight distribution. At Mugello, they raised the center of gravity by 20mm, a vast amount in a world where normally parameters are changed a millimeter at a time. At Laguna Seca, they tried another change, shifting the weight further back and leaving Rossi's bike looking more like a chopper than a racing motorcycle.
Indeed, the main rationale behind the switch from the GP11 originally fielded for the 2011 season and the heavily revised GP11.1 is that they could raise the center of mass much more without making the rear pump, a problem which the original design with the top-brace swingarm suffered from, and familiar to anyone who watched Casey Stoner coming out of corners on the GP9 and GP10 in previous years.
