It has been a busy semester. I’ve not found time to keep my blog up-to-date. Lots of things have happened with the EV project. During March a lot of time was spent researching suppliers and ordering parts. That’s also when spring break happened, so there was a one-week interval when nothing was done on the project. By the end of March, parts started arriving. Here are some of the big decisions that were made.
The students ran a Matlab simulation of the car to predict the energy needs and choose an appropriate battery pack and motor. The graph below shows current needed by the motor vs. time. The vertical axis is current from 0 to 450 A (50 A per tic). The horizontal axis is time in seconds (0 to 1400 seconds, 200 seconds/tic.) The simulation is for city driving with lots of starts and stops, a road speed limit of 30 MPH, and a few higher-speed intervals up to 55 MPH. The top speed is near 200 seconds in the simulation where you can see a peak current draw of about 410 A for a second or two. The average current draw was about 50 A as shown by the horizontal red line. The results below are just a sample of many graphs the the simulation produced. We expect about 20 to 40 miles of range on a charge depending on how the car is driven.
Lithium ion batteries would sure be nice, but the price is way too high. Since this is our first project we decided to go with something known and reliable: lead-acid batteries. The car will have twelve Trojan J150 batteries. These are 12 V batteries (6 cells per battery). Four will go under the front hood in a box mounted over top of the electric motor. We had to mount the transaxle lower than it originally was. This lowered the mounting location of the electric motor so that the hood will still close over top of the battery pack that is on top of the motor. Five more batteries will go under the back seat. More on that later. Three will go in the trunk. This gives a total of 144 V. The batteries can deliver more than 400 A but we will fuse them at 400 A. The total electric power available then is 57.6 KW or 77.2 hp.
The total energy stored in a fully charged battery pack will be about 45 MJ (megajoules). For comparison, a gallon of gasoline stores about 132 MJ, so the car will have the energy equivalent of about one-third of a gallon of gasoline in the battery pack when fully charged. Yet we expect at least 20 miles of range and maybe 40 miles. Thus the electric motor is much more efficient than a gasoline engine. We will have the equivalent energy efficiency of at least 60 MPG and up to 120 MPG when measured from battery output to the road. However generating the electricity and charging the car is only about 30% efficient. Overall from fundamental energy source (mine, well-head, hydroelectric dam, windmill, etc.) to the road, electric cars and gasoline cars are not much different.
We choose an Advanced DC Motors model FB1-4001A which we purchased from Electric Vehicles of America, Inc. This motor can deliver 30 hp continuously and up to 100 hp for a minute or two without overheating. The connection we plan to use will limit the peak power to about 77 hp. The motor is fastened to the existing 5-speed manual transmission by means of an adapter plate we purchased from Canadian Electric Vehicles. Ltd. We mounted the adapter plate to the motor, then put a hub on the motor shaft which allowed us to bolt the flywheel (removed from the gasoline engine) to the hub. Then the clutch module is bolted to the flywheel and the whole assembly is mated up to the transmission, just like a gasoline engine would be.
The role of delivering electric power to the electric motor is taken by a package of electronics called a motor controller. This device replaces the carburetor or throttle-body in a gasoline car. The controller we chose is manufactured by Curtis (model 1231C) and can handle up to 400 A of current. The accelerator pedal that formerly actuated the throttle plate on the gasoline engine will now be reconnected to a potentiometer mounted in a small metal box. This is called a “pot box” for short. The potentiometer is a variable resistance which sends a signal to the controller.
Under normal operation the controller turns the electric motor on and off about 15000 times a second. As you press harder on the accelerator the resistance of the pot-box element increases and the time during which the motor is on increases. In other words, if you press the accelerator slightly, for example about one-seventh of the way down, the motor is switched on for maybe 0.00001 seconds and then switched off. Just 0.000067 seconds later the motor is switched on again for 0.00001 seconds and this cycle repeats until the accelerator pedal is moved. So the motor is only on about one-seventh of the time. When the motor is off it (and the car) coasts. Since this is happening so rapidly, there is no pulsation or vibration. The motor just runs more slowly than if it was on all the time. As you press the accelerator down further the time on increases from 0.00001 to finally 0.00067 seconds out of each 0.00067 second interval (on all the time) when the pedal is all the way down. This strategy is more efficient than directly reducing the voltage to the motor (using a variable resistor for example) because the motor is not drawing any power much of the time.
1.) Inertia Switch
In case of a crash we want the high voltage section to shut down automatically so that the car cannot go out-of-control. We have an inertia switch for this. If there is ever a very large jerk or impact to the frame, it will be detected and the main contactor will automatically be de-energized until a button on the inertia switch is pushed. The inertia switch is not so sensitive as to react to ordinary pot-holes in the road however. (Most gasoline powered cars also have an inertia switch. It shuts down the fuel pump if there is a crash.)
2.) Secondary Contactor
The main power to the controller (and hence to the motor) goes through a part called a contactor. This is like a circuit breaker but it is remotely controlled by a low-voltage (12 V) signal. That signal comes from the key switch. When the key is “ON” or in the “RUN” position the main contactor conducts the high-voltage power to to the controller. When the key is “OFF” or in the “LOCK” position the power to the controller is blocked. But it is possible for the main contactor to fail in a “welded closed” mode, meaning power will not be shut down when the key is off. Because of this possibility, we added a secondary contactor. The secondary contactor is connected to a small switch on the pot-box. When the accelerator pedal is fully up, the secondary contactor also breaks the high-voltage circuit to the controller. Thus there are two fully redundant ways to shut the high-voltage power off. This also offers protection in case the controller fails in a full-power mode. (This failure would make the controller incorrectly act as if the accelerator pedal was floored.) The driver would instinctively let the accelerator pedal up and then the secondary contactor will shut the power off.
3.) Charger Interlock
It is possible for the driver to attempt to drive away with the charging power cord still connected. We added an interlock relay to prevent the secondary contactor from energizing if the car is plugged in for charging.
4.) Starter action
Sometimes people turn the key switch on their car to “ON” or “RUN” just to listen to the radio, but they do not actually start the engine. When this is done nobody would expect the car to move if the accelerator pedal was depressed since the engine is not running. A bunch of the dashboard warning lights (e.g. “oil pressure”) will be on as well, indicating that the engine is not running. We wanted to retain this behavior in the conversion. If you just turn the switch to from “LOCK” or “OFF” to “RUN” the “check engine” light will come on to inform the driver that the high-voltage system is not energized and the car will not be drivable.
To drive the car the key switch must first be turned from “LOCK” or “OFF” all the way to “START” and then released, from whence the spring load in the key switch will return the key to the “RUN” setting. Then the warning light will go out, the high-voltage system will be energized, and the car can be driven.
Since the contactors operate from the 12 V system that is energized when the key is at “ON” or “RUN”, this would energize the high voltage system immediately. To prevent that we added a relay we call the “feedback relay” in order to restore the starter action of the key switch. The feedback relay interlocks with the existing starter relay and prevents the “RUN” setting of the key switch from energizing the secondary contactor and the high-voltage system unless the previous setting of the key switch was “START.”
A note on the side
On a gasoline powered car using the “ON” or “RUN” setting of the ignition switch just to listen to the radio wastes battery power since the fuel pump will be running to pressurize the fuel injectors. The injectors will not open until the crankshaft rotates, so no fuel will flow, but the pump will be at work anyway, enabling you to start the car. Also the heater or air conditioner blower will probably be on, but no heat or air conditioning is available since the engine is not running. Other than the waste of electrical power this is not harmful to the car, but it is better to use the “ACC” setting for listening to the radio while the engine is not running.)
5.) Clutch and accelerator interlocks
When the car is started as described above, we do not want the car to lurch forward or backwards in case it was left in gear. To prevent that there is an interlock switch on the clutch pedal. The clutch pedal must be fully depressed in order to start the car. We used the same interlock switch that existed in the starter circuit of the gasoline engine for this. We also made a connection with the pot-box switch to prevent starting with the accelerator pedal depressed. The clutch pedal must be fully depressed and the accelerator pedal must be fully released to start the car. (Goosing the electric motor upon starting could damage the bearings. Since it will have no mechanical load it will very quickly spin up to thousands of RPM’s. Goosing a jet engine is a bad idea too, for the same reason.)
Back Seat Modification
The biggest issue that has turned up is the weight of the batteries, about 1000 lbs all together. We took the gasoline engine, fuel tank, exhaust system and so forth out removing about 400 lbs of weight. But the electric motor, charger, and other new electronics will add back about 150 lbs. (The charger will be on board so that we can charge at any 120 V outlet.) Thus we are making the car about 750 lbs heavier. Where we put the batteries will control where the center of gravity is and will affect handling. If we put most of the batteries in the trunk, behind the rear axle, steering and handling would be compromised. The weight behind the axle will tend to lift the front of the car off the road on bumps and curves.
Solving this problem has taken more time than any other single issue. Since the back seat is in front of the axle it seemed a good place for batteries, but the back seat is not high enough above the floor to just put batteries under it. We decided to cut a hole in the car where the back seat was and build a box for five batteries that extends down into the area where the gas tank formerly was located. Then we will rebuild the rear seat on top of the battery box. The photo below shows the hole being cut. Andrew is leaning in through the rear drivers-side door opening. (The doors have been temporarily removed to improve working access to the rear-seat area.) The camera is just outside the passenger side rear door opening. You can see the spare tire well in the trunk to the left and the rear floor carpeting and front seats all covered in plastic to the right.
As I write, the electric motor and the battery boxes are all in place and the car is ready for the high-voltage wiring. Just last week the students took the car out for a drive on the parking lot by connecting just one battery to the motor. We hope to have the high-voltage system and controller functional by Wednesday, May 6 for the senior project presentations.
There will be a public showing of the car during the senior project presentations on Wednesday evening, May 6, 2009 at the Dordt College Science Building. The presentations start at 7:30 PM in room S101. Other projects that will be presented that same evening are a tensile tester, A base-ball pitching and batting machine, and a shop crane.