Car Performance

This is the 4th post related to my car buying experience. The previous one was Buying a new car.

As I mentioned, my first criteria was a Hybrid. Hybrid cars give a good MPG and that’s attractive to some of us car buyers. They give better MPG by doing various different tricks. The most obvious one is to augment the conventional gas engine with a battery that delivers some power. Others are more mostly how these two systems are made to work together which I think is mostly controlled by software.

For regular cars, the city MPG is less than the high-way MPG. This is because of the stop and go pattern in the city and acceleration eats up a lot more gas than once traveling at a constant speed even at higher speeds. For hybrid cars however, the city MPG is usually listed to be more than the high-way. The reason is, when the speed is below 25 MPH or so, some cars are designed to just use the electric power for small distances. That means, there is no gasoline being used at all. Or, when the car stops at a signal or the traffic is very slow, the gas engine can completely shut-off. So, while the total energy used in cities is probably similar to regular cars and more than that on highways, hybrid cars give better MPG in cities using their battery power.

Another trick used by CT 200h is to have Eco mode where the throttle response is altered by making the driver to press the gas really hard. This may be perceived has negative by some. But CT 200h also has Normal and Sports modes where the throttle responses are perceptively better but sacrifice on efficiency.

When I decided to get CT 200h, I was already mentally prepared with the above negative points of slow response because my priority was efficiency. Once I bought the car, I experimented all the 4 modes (EV, Eco, Normal and Sports). My work to home commute is short and with lots of stops and traffic. So, I mostly drive in Eco mode during week days. Weekends, depending on the situation on the high-ways I adjust as required. I actually tried cruising around 68 to 70 MPH in Eco mode and it worked just fine (thought I am not sure if at those constant high-speeds there is much difference between the various modes).

One of the reasons the CT 200h feels slow is the horse power. It has a combined horse power of 134. I say combined because the gas engine just has 98 and together with the battery it gets 134 hp. One of the reasons I was fine with 134 hp was because my previous car also had the same hp. However, I think the combined hp probably still feels less responsive compared to the same hp for a regular car with just gas engine.

Given that there are cars with Horsepower in excess of 500, I wanted to understand what that means to the cars performance. As I understand the HP makes difference in two things

1) The acceleration time of the car
2) The maximum speed of the car

If someone is really into the fast acceleration, then they shouldn’t be buying hybrids or probably get high-end hybrids. But regarding maximum speed, that shouldn’t really be much of a concern. This is because, let’s see how the HP and velocity are related. As per Wikipedia article on drag,

 P_d = \mathbf{F}_d \cdot \mathbf{v} = \tfrac12 \rho v^3 A C_d

That is, power is proportional to cube of the velocity. Assuming all other factors (drag coefficient and the cross-sectional area) are the same for two different cars,

HP1/HP2 = (v1/v2)^3

So, if a car with 134 HP (like the CT 200h) has a max speed of 80 MPH (it’s actually above 100, but let’s not use extreme values), then a car with 350 HP will have a max speed of 110 MPH and a car with 500 HP will have a max speed of 124 MPH. All these numbers may not mean much but just realize that HP is not linearly proportional to speed. Given that the speed limits on high-ways are only 65 to at most 75 MPH, it probably doesn’t matter. Unless, of course if you want to tow stuff with your car.

With so many cars and options, it’s very difficult decide on a car. The best way to decide is to first decide on a few key important features and of course the budget. Then the choices narrow down drastically and with a bit more of online research, it should be possible to pick up to three cars. Test driving them should narrow it down to 1.

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