Hybrid Clutchless Transmission

We developed a patent-pending approach to hybridization that alleviates concerns around inconvenient charging, weight, and top-speed performance of existing electric (EV) and hybrid-electric vehicles (HEV). Our clutchless dual-shaft hybrid architecture implements a single electric motor (EM) and internal combustion engine (ICE) in their optimal range of use to enhance the acceleration performance and efficiency at low speeds without sacrificing range, top-speed, and driving experience. Additionally, removal of the clutch and initial gears reduces frictional losses, weight, and size of the powertrain. Advantages of the proposed architecture over existing alternatives - specifically for high-performance vehicles - were theoretically proven, and bench-level feasibility was demonstrated.

Our objective moving forward is to construct and fit a second-generation prototype into a go-kart to validate functionality under realistic loading.

Contributions

  • Concept Generation
  • Vehicle Performance Modeling
  • CAD
  • Prototyping

Collaborators

  • Victor Prost
  • Zackary Eubanks
  • Daria Bondarchuk
  • Paige Reiter
  • Yu Hua
  • Daniel Dorsch (Mentor)

Conference Publication Prototype CAD Patent Pending

Motivation

Greenhouse gas emissions have approximately doubled from the transportation sector since the 1970s, accounting for approximately 14% of all anthropogenic sources. Of that total contribution, approximately 72% is attributed to road transportation [1]. In response to the threat of climate change that this sector poses, governments have begun to regulate fuel efficiency and impose stringent emissions requirements for new automobiles through programs such as the Corporate Average Fuel Economy (CAFE) initiative, or by completely banning purely Internal Combustion Engine (ICE) powered vehicles [2].

In an effort to meet these standards and reduce the environmental impact of their vehicles, automotive manufacturers have gravitated towards electrification and hybridization of powertrains. However, while the popularity of electric (EV) and plug-in hybrid vehicles (PHEV) has increased substantially over the past decade [3], they have captured only 0.86% of the market [4]. Existing technology suffers drawbacks that include short range, limited speed, high cost, and inadequate charging infrastructure [5, 6, 7]. In addition, some customers associate the technology with compromised driving experience [8].

To improve adoption and successfully capture this increase in efficiency and emissions reductions that hybridization and/or electrification promises, the above concerns need to be alleviated. We believe that this objective can be achieved through the development of novel automotive power transmission architectures. For this work, we specifically targeted the high-performance sector, and developed a hybridization concept that can improve efficiency and emissions characteristics at low speeds (typical of city-driving) without sacrificing the top-speed and acceleration performance expected from a high-performance car on the track.

The Idea

An internal combustion engine (ICE) typically produces peak torque at high speeds. As a result, a vehicle powered by an ICE consists of a transmission with several gears to allow operation at a broad range of speeds. The first plot on the right provides an assumed torque curve for a 7-speed rear-wheel drive performance car. Each curve represents a different gear ratio, with a maximum speed in first gear of 77 km/hr at the engine’s red-line speed of 9000 RPM.

In the next plot, I have added curves to represent the frictional torque due to rolling resistance and aerodynamic drag, and the maximum torque that can be sustained by the wheels (the tire friction limit). From this figure, it is clear that performance is maximized by increasing the difference between the available (solid navy) and frictional torque (dashed red), in the region below the tire limit (dashed red). One can then analyze this torque "envelope" for an ICE-driven vehicle and identify key regions where improvements could be made by adding an electric motor (EM).

Peak EM torque occurs at low speeds, making it most suitable for lower-speed driving. This approach allows the ICE gear ratios to be decreased for greater torque at higher speeds, or the first one/two gears to eliminated for weight reduction purposes. Such a strategy promotes more efficient and environmentally-friendly operation in the city (<100 km/hr in this example) without sacrificing high-speed performance.

Physical Architecture

Illustrated here is the proposed embodiment of a hybrid transmission architecture that implements an EM and ICE in their optimal ranges of use. It features two power sources, dual shafts, and no clutch.

Inspired by a dual-clutch transmission (DCT) system, this architecture features even and odd gears on two separate shafts. At the input, synchronizers A and B allow either or both EM and the ICE to provide power to the output through either shafts 1 or 2. Benefits of this arrangement include:

  • Power-launch: Both EM and ICE can deliver power simulatenously to the output to provide high lauch acceleration rates.
  • Torque-filling: The EM can provide torque to the rear wheels during the ICE shift sequence to minimize jerk and prevent the vehicle from losing too much speed.
  • Speed-matching: The EM can be used to speed-up or slow-down the ICE to facilitate fast gear-shifts.
  • Efficient city driving: Low-speed EM operation facilitates improved efficiency and emissions performance in the city. The ICE can be operated at its most efficient RPM range to recharge the battery when required.
  • Reduced transmission weight: Removal of the clutch, reverse gear, associated idler, and up to the first to gears is expected to reduce transmission weight and size.

In a traditional transmission, the primary function of a clutch is to handle large speed differences between the ICE and transmission during gear shifts. In our concept, this function is handled by the car’s ECU and EM during speed-matching and torque-filling operations. Detailed walkthroughs of the shifting strategies are provided in our conference publication.

Performance Modeling

The 0-60 MPH (100 km/hr) is an important metric for high-performance vehicles. In order to launch the car using a traditional transmission, the engine is first revved at stand-still with the clutch disengaged. When the clutch is suddenly engaged, kinetic energy from the engine’s flywheel is used to accelerate the vehicle. In comparison, the launch sequence for this concept is outlined as follows:

  1. With first gear engaged, the EM begins to accelerate the vehicle through Shaft 1 while the ICE idles at a fixed speed.
  2. Synchro B is engaged when Shaft 1 matches the speed of the ICE (accounting also for gearing), at approximately 22 km/hr in this example. Both ICE and EM now drive Shaft 1.
  3. The ECU regulates the torque to remain just below the tire friction. The EM does not make significant contributions to total torque as its maximum speed is approached. It is subsequently disengaged while the ICE continues to accelerate the vehicle past 100 km/hr.

Using Matlab, I quantified the performance benefits of this approach by simulating the addition of a Brusa HSM1 180 kW Hybrid Motor [9] to a Ferrari 458 within the proposed hybrid configuration. Gear ratios for the EM and ICE were optimized to eliminate the need for shifting between 0 and 100 km/hr while providing maximum acceleration: 2.80 and 1.75, respectively, with a final drive gear ratio of 5. The velocity and torque plots (below) indicated a 0-100 km/hr time of only 3.18 s with no breaks in acceleration! Furthermore, the proposed concept has the additional advantage of preserving the visceral (noise and vibration) experience of driving a supercar.

Prototype Design and Fabrication

We designed and constructed a bench-scale prototype to demonstrate that the hybrid launch and shifting operations could be performed without the use of a clutch. The prototype was constructed using select shafts, gears, and synchronizers from three identical 1999 Honda Civic transmissions mounted on a 4 ft custom aluminum and Delrin test bed. Constrained by the parts that were available, the prototype incorporated an idler shaft as a link between the two motor input shafts. In addition, only four out of the six gears were implemented in the prototype as this allowed for adequate demonstration of all key functions. Both input shafts were powered by the electric drives from two Craftsman C3 drills, and four 5 V servos were used to shift the synchronizers. The system was powered by a DC power supply (10A, 30V) and controlled by an Arduino Mega micro-controller, a custom speed control circuit, and two Hall-effect sensors.

Prototype Videos

Videos demonstrating the launch and shifting sequences, using the prototype are provided below.

References

[1] Edenhofer et al. “IPCC, 2014: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.” [2] Staufenberg. "Norway to ban the sale of all fossil fuel based cars by 2025." The Independent. June 2016. [3] Alternative Fuels Data Center. Accessed July 2017. [4] The Electric Vehicle World Sales Database. Accessed May 2017. [5] Rinkesh. "What is a Hybrid Car?" Conserve Energy Future. Accessed July 2017. [6] Richtel. " In California, Electric Cars Outpace Plugs, and Sparks Fly." The New York Times. October 10, 2015. [7] Needell et al. "Potential for widespread electrification of personal vehicle travel in the United States." Nature. August 15, 2016. [8] Rauwald and Rocks. "The Electric Porsche Needs to Roar." Bloomberg. November 11, 2016. [9] BRUSA Elektronik AG. "HSM1 Hybrid Synchronous Motor."