It seems that new engine technologies are bring released by carmakers at an ever-increasing pace and while some new technologies are true innovations, others are mere adaptations of existing technology that has been around for decades. One such technology, the much talked about Budack Cycle that was developed by Volkswagen, is a good case in point. In this article, we look at what the Budack Cycle entails, how it works, and what its advantages are, starting with this question-
To understand the rationale behind the Budack cycle, we need to briefly look at the two predominant cycles that govern the operation of 4-stroke internal combustion engines. The first is the Otto cycle that works on the principle of an intake stroke, a compression stroke, a power stroke, and finally an exhaust stroke that expels the combusted air/fuel mixture from the cylinder. We are all familiar with this system, and we know that with the Otto cycle, the intake valve closes when the piston cannot “suck” more air into the cylinder. However, there is a well established variation on this system, known as the-
Miller Cycle
On engines that use this cycle, the intake valve closes later than on an engine using the Otto cycle, thereby pushing some of the air/fuel mixture back into the intake manifold runner/plenum. The advantage of this is that less energy is wasted as heat during the compression stroke, since there is a smaller volume of air and fuel to compress, but this comes at the cost of reduced power as compared with the conventional Otto Cycle, since the effective compression ratio is reduced.
While the Miller cycle is in widespread use today, one Volkswagen drive train engineer, with the name of Ralf Budack, devised a way to reduce the negative effects of the Miller Cycle by developing his own cycle, hence the name, “Budack Cycle”. This cycle saw its first use on the 2.0 L TSI EA888 Gen3 B-Cycle engines fitted to the 2017/18 VW Tiguan.
Essentially, the Budack Cycle is the opposite of the Miller Cycle, in the sense that the Budack Cycle closes the intake valve sooner than it would have closed on a conventional Otto Cycle engine. While this hardly qualifies as a technical innovation, it does have two distinct advantages over both Otto and Miller cycle engines, these being-
Improved combustion
The practical advantage of closing the intake valve early is that since the piston is still travelling downward when the valve closes, the air/fuel mixture is “stretched”, which improves mixing of the air and fuel, thus improving both combustion and fuel economy.
However, to maintain an effective compression ratio of 11.7:1, Volkswagen has developed special pistons with pronounced bulges on their crowns, which further promotes mixing of the air/fuel mixture. The combined net effect of the intake valves closing early and the design of the pistons is that a larger effective combustion chamber is created, resulting in a higher flow rate of the incoming air/fuel mixture. In turn, the improved airflow improves combustion, which in its turn produces a (claimed) 8% increase in power over the 2.0L EA888 engine that does not have the benefit of the Budack Cycle.
Improved driveability
The main objective of Volkswagen’s policy of “rightsizing” their new engines is to improve fuel economy and overall driveability of their new engines mainly in the range of operating conditions where any given new engine will be used the most often. In the case of the 2018 Tiguan, those conditions are judged to be mostly in the urban cycle, where maximum torque and power are required at lower engine speeds.
As a result of the Budack Cycle and other refinements, the EA888 Gen3 B-Cycle engine now develops it maximum power of 134 kW at 4,400 RPM, which is sustained until 6,000 RPM. Maximum torque of 299 Nm is developed at only 1,600 RPM, but is maintained until 4,300 RPM, which makes this engine eminently suitable for city driving.
Nonetheless, the Budack Cycle engine comes with other refinements that include-
Variable valve lift and duration
While there is not much information available on the actual control mechanism, what is known is that the valve lift is closely linked to the throttle position. For example, at low speeds and light engine loads, the intake valves are not opened to their maximum lift, which contributes to the engine’s volumetric efficiency and fuel economy. Conversely, at high engine speeds and loads, intake valve lift is increased and the inlet valves are closed later, thus switching the engine over to normal Otto-cycle operation.
It is perhaps worth mentioning that the Budack Cycle engine uses smaller valves than non-Budack Cycle EA888 engines, and according to official sources, this is possible because maximum gas flow through the cylinder head is not a concern at high engine loads. However, there is no technical information available on why maximising gas flow is not a priority on this engine, or on how efficient the exhaust manifold/system is at extracting exhaust gas from the cylinders.
High-pressure direct fuel injection
According to official sources, the cylinder head of the Budack Cycle EA888 engine was redesigned to allow the fuel injectors to be placed closer to the combustion chambers. Fuel injection pressure was raised to 250 Bar to improve fuel atomisation, while depending on operating conditions, injection events can be split into up to three separate events during a single intake stroke.
Most of the other refinements to the Budack Cycle engine were made with the aim of reducing weight and friction, which when taken together, make a significant contribution to the overall efficiency of this engine. Briefly, these improvements include the following-
NOTE: Brake Mean Effective Pressure is defined as the average pressure, which if it were imposed uniformly on the pistons during the entire length of the power stroke, would produce the engine’s measured power output. Note that this value is purely theoretical, and has absolutely nothing to do with the actual pressures that occur within a cylinder during combustion. This value is simply a mathematical tool engineers use to describe how efficiently an engine with any given displacement produces torque.
