Due to the long existence of internal combustion vehicles, we are familiar with their infrastructure. As soon as we see a car, we know the vehicle will have a fuel tank where the fuel is stored. This fuel is then pushed into the engine with a fuel pump. The fuel at some point mixes with the air and with the help of a spark in a gasoline engine or compression in a diesel engine produces combustion inside the cylinder. Due to the combustion being inside the cylinder, it is called Internal Combustion Engine or ICE. This combustion pushes a piston converting a longitudinal motion into rotation by rotating a crankshaft. This rotational energy goes through a transmission to get the right speed and torque conversion to get a speed at the wheels. The transmission be it an automatic or a manual helps convert a limited engine operation range of 0-8000 RPM for gasoline or 0-2000 RPM for a heavy-duty diesel engine into continuous rotation at the wheels of the vehicle.
Things operate a little differently for a Battery Electric Vehicle (BEV). For one, you don’t have an engine and the source of energy is not fuel but electrochemical energy. Let’s imagine building a BEV from scratch. In the simplest terms, for an ICE to work, you need air, fuel, and spark to go from raw fuel energy to energy in usable form. For a BEV, you would need a battery, electric motor, and wires to connect them. As simple as an RC car. That’s it. The complexity comes when we are dealing with driving heavy vehicles and needing to operate for longer durations of time and operating driver comfort and auxiliary components like cooling systems and cabin conditioning which usually operate at different voltage levels. Very similar to how a phone operates at 4V (approx.) and your laptop operates at around 15V (approx.). I categorized the main differences between a BEV and an ICE into the following heads:
1. Energy Source – Fuel vs Battery
2. Power Unit – Engine vs Motor
3. Auxiliary power
4. Refueling – Refueling vs Recharging
Energy Source
The source of energy for an ICE is our very well-known fuel. Fuel is obtained by processing natural gases developed due to pressure from the layers of the earth on the fossils of early earthlings over a time of about millions of years. We mainly see gasoline and diesel used in vehicles primarily. The only difference between them is their density. Hence even if they both have about the same calorific value (combustion energy), diesel can produce higher power with the same amount. Some additives are added to the fuel, sometimes mandated by the regulation authorities that help in emission reduction and lubrication inside the engine ensuring improvement in engine life.
The energy source in a BEV is the electrochemical reaction within a cell. A cell comprises of an anode (-ve), a cathode (+ve) and a separator/electrolyte. The anode and the cathode are connected through the separator inside the cell and a “load” on the outside of the cell. The idea is that, on the application of a load, the Lithium (Li+) ions travel from anode to cathode through the separator and to balance the +ve charge, electrons (-ve) travel from anode (-ve) to cathode (+ve) through the load. The load in this case is an electric motor. Different elements like Cobalt, Nickel, Manganese etc. are added in different proportions to improve the performance of the battery. For example, a cell chemistry of Li Iron Phosphate (LFP) is known to be power dense (better acceleration) and a cell chemistry of Ni Manganese Cobalt (NMC) is energy dense (Longer range).
While we measure the fuel capacity in gallons, the battery capacity is measured in Kilowatt-hours (kWh) which is a unit of energy. A Tesla Model 3 can have an 80kWhr battery as compared to a Honda Civic that has a 12.5 gallons of fuel tank. Both claim to provide a range of 300 – 400 miles. A heavy-duty truck like a Kenworth T680 can carry about 270 gallons of diesel with 2500 miles while its electric counterpart the T680e is designed to hold 400 kWh and is claimed to give 150 miles depending on application.
Power Unit
The power unit for an ICE vehicle is like the name suggests ICE. It is called so because the combustion that produces power takes place inside a cylinder. It is usually a 4-stroke (sometimes 2-stroke) cycle with intake, compression, power, and exhaust strokes. As soon as we apply the accelerator pedal, there is a preset calibration inside the vehicle controller (ECU – engine control unit) that maps the pedal position to a torque demand value from the engine which usually correlates with the amount of fuel and air supplied to the engine. The higher the torque, the higher the power.
The BEVs are similar, the accelerator pedal position is now mapped against torque which however is mapped against battery current demand. There are two levels of torque that a driver can request from the motor. This is peak torque and max continuous torque. The motor can deliver peak torque only for a few seconds. This is why you see electric cars having insane acceleration. But this power is turned down to protect the motor from getting damaged to a maximum continuous torque value.
While this is not common in smaller IC engines, there is a concept of engine retarder braking or Jake braking in trucks. In this type of brake, the vehicle uses the compression of the air (only) inside the engine to provide additional resistance to speed. This is important because sometimes trucks can have nearly 80,000 lbs. of moving mass, which makes it impossible to slow down on a slope. You might be pressing the brakes as hard as possible, and the vehicle is still accelerating if the engine brakes are not applied. For the BEVs, the equivalent is regenerative braking. This is when the vehicle uses its speed to charge the battery backup. This is also possible in smaller cars. An important detail here is that, if a vehicle goes up a hill and then regenerates on the way back, even though the up slope and down slope are about the same (not always, but let’s assume), the vehicle does not regenerate equivalent amount of energy that it spends going up. This is because of a multitude of reasons. (1) The speed of deacceleration determines the rate of charging (2) there is a cap to the power the battery can be charged with (ballpark - 35% of the peak power) (3) there are losses in the form of regeneration efficiency which is lower than the discharging efficiency of the battery. Speaking of efficiency, an ICE vehicle operates at 30% – 40% efficiency, it’s normal for a BEV to operate at 85% efficiency.
Auxiliary Power
Any component that does not add to propulsion is called an Auxiliary. The air conditioner, water pump, power steering etc. are all auxiliaries that draw power and need energy to function. For an ICE vehicle, these components are driven by the engine rotational energy through a driving belt. This is a black color (mostly) belt with one side that is smooth, and the other side rough and should usually be very easily identified on popping the hood open unless it is hiding behind a panel. This driving belt also known as the serpentine belt connects the crankshaft to the water pump for the oil and coolant in the engine cooling system, the AC air conditioner compressor and sometimes the hydraulic power steering pump. Sometimes the hydraulic pump is replaced by an electric power steering system which draws the power from a 12V battery.
On the other hand, in a BEV, the components remain the same, however, now they can be electronically powered. This is not just to match the source of power in the BEVs, but also having an electric actuation allows for a wider range of motion; and smoother, better controlled and more efficient operation. Some of the components are at different voltage levels than the 12V battery and the 300 – 600 V population battery pack. Some of these components operate at 24V going all the way to 48V more in special cases. Given this disparity, the vehicle needs to be equipped with a DC/DC converter and an investor in case some of the auxiliaries operate in AC.
The cooling system in an ICE is for the engine and the transmission. In a BEV however, sometimes the transmission and the motor are 1 unit. The cooling is hence required by the motor block. The battery also requires temperature moderation to perform optimally and preserve health. An unhealthy battery can lose up to 30% of its range. Temperature moderation refers to cooling as well as heating. The power electronics also at times require cooling but often come with metal fins which are flat high surface area extrusions that help cool the box via heat convection into air.
While auxiliary power usage in ICE wasn’t a major concern, in BEVs it is very important given that it uses battery energy that would have been used in driving instead.
Refueling and Recharging
Battery energy inside the vehicle is a scarce resource since it would take hours if not minutes to put energy back into the battery pack. This severely impacts the freedom drivers get while driving their vehicles and causes “range anxiety”. Refueling an ICE is very straightforward now. You drive up to the gas pump, decide which octane-rating fuel you want to get, wait for 10 minutes for your tank to refill and be on your way. Things aren’t so straightforward when it comes to recharging a BEV. There are very few charging stations today to start with, however, there have been plans laid out by the government to change that and soon enough it will be different.
There are 3 levels of charging available in the market today. Level 1 is a 120V charger which can plug into the home charging ports. Level 2 is a 240V charger that is popularly available in public chargers today. Both use the same type of charging connector standardized as a J1772 plug. Level 3 or Direct Current Fast Charging (DCFC) is the most powerful charger now and can go up to a 1000V charging system. Charge always flows from a higher potential to a lower potential. Hence, it’s hard to imagine a 120v or 240v system being able to charge a 350v car battery. But this charging is made possible with an onboard charger which is an inverter that increases the voltage. Since energy can neither be created nor be destroyed, the increasing voltage is compensated by decreasing current (Power = Voltage X Current). Hence, the higher the voltage must be increased, the lower the current ends up being. The speed of charging depends on the amount of charge ( or current ), hence lower current implies longer charging time.
Anywhere in the spec sheet for the BEVs, it will be mentioned that the vehicle is capable of charging from 10%-80% in x minutes. This begs the question of why not 0-100%? This is because of a characteristic curve of the battery called the State of charge (SOC) vs Open Circuit Voltage (OCV) curve and the internal resistance of the battery cells. (For less than 10% SOC there is a shape rise in OCV with SOC, As SOC increases after 10%, the OCV also rises but not as sharply until the SOC reaches about 80%. A very sharp OCV increase for SOC increase until 100%.) With a small increase/decrease in SOC from 0%-10% and 80%-100%, there is a large change in OCV, hence it is quite possible that going from 10% - 80% would take about equal or less time than going from 0%- 10%. At the same time, the internal resistance of a battery depends on SOC as well, between 0%-10% and 80%-100% the internal resistance is much higher than it is between 10%-80%. This slows down the charging tremendously as well. (This internal resistance is a necessary evil since the absence of it can cause large short-circuit sparks and damage to the user and the device.)
There are a lot more differences between BEVs and ICE vehicles in terms of drivability, user experience, insurance, and vehicle life. This blog captures an engineering-level difference in the four main categories with a little bit of insight on each. BEVs will be the future, but as of today and in the short term, they are not the solution for sustainability. Engineering must evolve as well as infrastructure to make BEVs a reasonable alternative which takes time and time can’t be accelerated.
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