4. Dartmouth’s Formula Hybrid design strategy

Last week I introduced you to Dartmouth’s Thayer School of Engineering Formula Hybrid team and discussed the lessons the program teaches about teamwork. This week we take a look at the team’s design strategy for their car which carries the number 007 because the team finished seventh in last year’s fourth annual Formula Hybrid competition. They call their little black machine ‘Moneypenny’.

Dartmouth’s car is a parallel hybrid drivetrain design. It’s powered by a 250cc Honda CRF250X moto-cross engine which makes 27 hp at 10,500 rpm plus an HPGC AC-15 electric motor in parallel which produces 43 hp. The team has moved this year from a carbureted gasoline engine to a more modern unit with electronic fuel injection. Electrical power is accumulated in a package of 40 Maxwell ultra capacitors. “Our whole strategy,” commented team leader Eric Mann, “is limiting the size of our accumulators and having fast-in and fast-out energy.”

When the Formula Hybrid competition started, Dartmouth’s team ran their existing Formula SAE cars which they turned into hybrid cars. But two years ago they built a brand new, purpose-built hybrid car that they have rapidly developed.

“Our current chassis is the first from the ground up hybrid car that Thayer built,” Mann explained. “We had an engine that would run a generator and all the power ran in series from the generator to the accumulator to the electric motor and out. But last year we switched to a parallel architecture where you could have the gas engine or the electric motor or a combination.

“We have the parallel hybrid drivetrain which is the engine and the electric motor. So there’s an engine team and a rear electric motor team. We have a vehicle cooling team and then we have driver controls. As we got into the hybrid there are so many more controls.

“We started over last year and there were some big failures but this year we’re lucky enough that it’s working reliably. We’re going to keep the same architecture so all the information we’re going to get now will just make it that much better.”

Frank Fortin-Houle says the team is closing in on finding the best overall package. “We’re slowly converging towards a solution,” Fortin-Houle said, “which makes it easier to focus on certain areas as opposed to the whole car.”

Added Mann: “I think in the competition as a whole in the first year everybody started with their own ideas and some things worked and some things didn’t. Bigger schools are coming in and becoming attracted to the competition so it’s definitely pushing it in the right direction. It’s not that all the teams are converging to one solution but they’re definitely becoming more refined.”

Philly Croteau agrees with Fortin-Houle and Mann. “It’s definitely moving from being the new hybrid concept to an actual race car that’s a hybrid as opposed to having the science project aspect of it,” Croteau observed.

One of the challenges of a Formula Hybrid car is coping with the high voltage inherent in a hybrid and making sure the car is as safe as possible. “That’s probably the most rigid part of the rules in Formula Hybrid,” Mann commented. “There’s a lot of constraints and safety checks. Everything has to be in conduit and be isolated if it’s over thirty volts. There are a lot of kill switches so everything can be killed from the outside. Any time the high voltage system is on there’s a light that’s blinking.”

Added Adam Marano: “There’s a ground fault detector and it won’t even let the high voltage system or anything on the car power up until it’s had a chance to verify the wiring integrity. If it detects that there’s any significant difference between the high voltage and low voltage side it just won’t let the car power up.

“Except for the huge capacitors all the motor controls are bled off within ten seconds of when the car turns off so you’re not keeping any stray high voltage. We’re monitoring voltage and temperature. Every wire on the car has to be fused to its gauge so it can’t spark fires and burn somebody.”

Croteau says the team of people working on cooling the car’s components are critical to both safety and performance. “Their job is, number one, to get the engine cooled and the rear electric motor cooled and the motor controller and brakes cooled,” Croteau remarked. “They then develop an efficiency model for the car. One of their goals is to calculate what is the loss from the allotment of energy in the brakes, the engine and the electric motor controller and also what’s the loss in the rolling resistance.”

Dartmouth’s car differs from many Formula Hybrid entries in having its batteries located low and close to the car’s centerline. “Our car is quite wide,” Fortin-Houle observed. “We’re the only team that runs with the batteries inside the car. A lot of teams have very narrow cars but with large battery banks sticking out. Our idea was to try and bring the batteries as close as possible to the driver and keep the weight down the centerline because the batteries are the heavy part.”

Fortin-Houle pointed out that the team has been able to cut a lot of weight out of the old Formula SAE-based car. “Our car from three years ago went from weighing almost a thousand pounds to weighing 570 pounds,” Fortin-Houle said. “So we’re learning how to make hybrid race cars, not just hybrid cars.”

Added Mann: “A hybrid is primarily about packaging. You’re trying to drag down the weight and fit it all in there. The big teams on the big race circuits obviously run into packaging issues too as they try to reduce weight.”

The team is reasonably happy with the handling of their car. “The SAE cars were inherently pretty narrow so the opportunity to build one from scratch enabled us to build a better hybrid car,” Mann commented. “We’re pretty happy with how it handles and the suspension geometries. That part isn’t broken so there’s no reason to change it. So we’re going to keep the chassis and suspension essentially the same. The amount of development that goes into a hybrid car means it’s too hard to build every year from the ground up.

“We’re dedicating so much to the powerplant. We’re making some jumps with the electronic fuel injection but we’re keeping the suspension. We’re starting to tune it and try to get the car handling better and understand how our systems work and have it be really robust. That’s what our overall goal is.”

Christian Busch is focused on making lighter and better suspension. “I’m doing computer simulations,” Busch commented. “We measured the forces with strain gauges and for next year’s car we won’t have to test the geometry. Building new A-arms cost a lot of time and money and when you have the simulation you can compare the results so you can use it for faster and more effective building of the suspension.”

Under Busch’s direction the team will move from using traditional steel A-arms to carbon fiber wishbones. “We purchased three main carbon fiber tubes and Christian makes all of the inserts for the geometry,” Mann said. “The big part has been the manufacturing process of getting good adhesion between the metal insert and the carbon fiber tubes.”

The new carbon fiber wishbones won’t be ready until later this year. “We have some assembled but we’re not done with the simulation,” Busch remarked. “There may be a change in geometry or design so we won’t build the final pieces until we have completed all our simulation work.”

The springs and shock absorbers on Dartmouth’s car are operated by pull rods which require more support in the lower part of the chassis. “Compared to Formula One and Indy cars we go so slow that aerodynamics does not have a big impact on our cars,” Fortin-Houle observed.

“Aerodynamics is such a big influence in modern race cars that everybody in most major Formulas are running pushrods so the nose is quite high and you can push a lot of air under the car. But we don’t go fast enough to have an aerodynamic impact so our cars look a lot more like older race cars. In that sense our cars are mechanically sound rather than aerodynamically sound.”

The main Formula Hybrid event is an endurance event where you’re given an initial allotment of energy. Each team is given more or less fuel depending on how much capacity is in their electrical system and how many batteries or capacitors they have. More electrical power means less fuel allotment.

“We’ve optimized it to the point where we have enough electrical power to be able to do the electric-only acceleration event,” Croteau pointed out. “But we’re maximizing our fuel for the endurance event which is where the majority of points are awarded for the competition. We’re trying to develop our electric system and our engine to the point where they work consistently and are very efficient.

“Rather than trying to tune the carburetor we’re going to use the electronic fuel injection to change our fuel-mapping and hopefully do that on the fly through the four different events. We’ll be able to change the fuel map for the acceleration event versus the endurance event to really optimize our performance.”

Added Mann: “It’s a choice of whether you’re going for performance or efficiency. The autocross is one lap as fast as you can go so for that you could have more performance-based fuel mapping. The endurance event is about efficiency and trying to get the best possible range out of it.

“Our overall strategy is to have fast-in power with regenerative braking and try to recoup some of the losses that you get out of the hydraulic braking. We went with capacitors rather than batteries to get fast-out power because the old technology batteries are current-limited, although the latest batteries are definitely getting better.”

Mann added that regenerative braking makes driving difficult. “Last year we got into regeneration and acceleration on the paddle control,” Mann said. “You have the accelerator for the gas and your main mechanical brake and then your clutching and shifting. So the amount of things made it a little challenging to drive.”

Fortin-Houle also emphasized the difficulties in using the braking regenerator. “It’s hard to optimize regeneration while you’re driving,” he commented. “When you’re coming into a corner you have to downshift through the gears and at the same time you have to operate the regenerator with the same hand you’re using to downshift. You have to commit to one or the other and because you have to downshift regeneration takes a backseat. So we’re trying to incorporate the regenerator with the brake pedal so that every time you’re braking you’re regenerating.”

Added Mann: “The reason we initially decoupled it was because you want to regenerate as much as possible and not use the mechanical brake so you’re not losing efficiency. We weren’t far enough along in the development to control it electronically through the car’s brain so it was the best way to maximize our efficiency.

“But when you see the complications that arose the driver is not maximizing it. So we’re definitely getting the driver controls more integrated especially with the EFI. We’re looking at electronic shifting by spark cut, or at least wire shifting that’s pneumatic or electronic, or paddle shifting on the steering wheel.”

Mann says the complexity of the car can easily get in the way of competing successfully. “One big thing we struggled with in the past is there are so many complex systems and the development pushes up so close to the competition date,” he observed. “So we’re always struggling to drive. There’s so much stuff that’s so new to a lot of people that it almost feels like a science project.

“This year our goal is to make the jump to having it be a race car and get to where we’re testing and understanding everything about it so you’re not making such huge strides in new technology but you’re really starting to optimize and find efficiency.”

As I said last week, there are plenty of real world engineering lessons to be learned by competing in a fascinating and demanding contest like Formula Hybrid.

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