Economist Writing Every Day

My old Honda Civic was a fairly small sedan. It had a 1.8 liter engine, that generated about 140 horsepower. It would not win any drag races, but had functional acceleration.

I recently got a Honda CR-V, a much larger, heavier vehicle. I was nonplussed to learn that it only had a 1.5 liter engine. Would I have to get out and push it up steep hills? As it turns out, this small engine can crank out some 190 horsepower. Given the size of this crossover SUV, this still does not make for a peppy drive, but at least I can actually pass another car as needed.

This high power with small engine displacement is made possible by the magic of turbocharging. As the (hot, expanded) exhaust gas leaves the engine, it goes through a turbine and makes it spin. A connected power shaft then spins a compressor, which takes outside air and jams it into the engine at higher pressure, i.e., higher density. With extra air stuffed into the engine cylinders, the engine can inject extra gasoline (keeping the air/fuel ratio roughly constant) – -and voila, high power output.

A schematic of this setup is shown below. I drew in a red arrow to mark the exhaust turbine, and a blue arrow for the intake air compressor. The rest should be fairly self-explanatory.

Source: Wikipedia

Also, here is a diagram of what the actual turbo hardware might look like:

Source: TurbochargersPlus

When the engine is turning at low-moderate speeds (say below 2000-3000 RPM), the turbine is doing relatively little, and so you are essentially driving around with a small (1.5 L) engine. This is good for gas mileage. When you floor it, the engine spins up and the turbo boost kicks in, giving considerably more power. [1]

What’s not to like? Apart from the potential maintenance headache of a rapidly spinning, complex chunk of precision machinery, there are a couple of issues with driving turbocharged engines that drivers should be aware of. There are articles  and videos (see good comments there) that address these and other issues in some detail.

( A ) Time Lag Before Turbo Boost Kicks In

With a normal non-turbo engine, you can feel the power kick in nearly immediately when you depress the pedal. The pedal opens the throttle, and instantly the engine is gulping more air (and fuel).

With a turbo, there can be a detectible time lag. The engine must rev up until the turbo effect starts to kick in, and then it spins faster, and there is more air shoved into the engine. As long as you know this, you can drive accordingly. This might be a life and death matter if as you are in the middle of passing a car on a two-lane highway, and suddenly an oncoming car appears in your passing lane. If you are not up to full power by that point, such that you can complete the passing quickly, you could become a statistic. I have only faced that situation maybe once every ten years in my driving, but it should be figured in.

The actual time lag varies from one model to another. I’d suggest just testing this out on your car. In some safe driving scenario, floor it and assess how much of a lag there is.

( B ) Don’t turn the engine off immediately if it has been running fast.

The thought here is to let the engine slow down to idle, and maybe even cool down a hair, before turning it off. The reason is that if the engine is revving at say 2000 rpm, and you suddenly turn the engine off, the oil pumping action stops, but the turbo is still spinning away in there. Having the turbine spinning away with no oil circulation can wreck the bushings.

There are articles  and videos (see good comments there) that address these and other issues in some detail.

Comment on Driving Honda CR-V Turbo Engine

Various engines have been used in CR-Vs. The 1.5 L turbo has been common in North America since 2017. It was designed to not have a very noticeable lag, in the sense that nothing happens for two seconds, and then the vehicle lurches forward. The turbo effect reportedly starts to kick in at 2000 rpm. However, this effect is progressive, so the power at 2000-3000 rpm is still modest. So, if you just push halfway down on the accelerator, the response is modest. If you floor it, the engine will within a second or two scream up to like 5000 rpm, and then start to really accelerate. That said, I have a visceral aversion to revving my engines that close to the red-line danger zone on the tachometer (my previous non-turbo cars I never took above about 3500 rpm, never needed to). Even with all that revving, the net acceleration is still modest.

Another factor with driving a CR-V is the “Econ” fuel-saving engine setting. When that is on, it seems to prevent the engine from revving over about 3500 rpm. So, if I plan to pass another car, or if I need power for some other reason, I need to remember to punch the leafy green Econ button to turn off this mode.

The bottom line is that I will think twice, maybe thrice, before passing another vehicle on a two-lane road in my CR-V.

ENDNOTE

[1] That is the theory anyway: great gas mileage most of the time, and bursts of power available for those rare times when you need it. The reality seems to be a little different. There may be reason to believe that turbocharged small engines give good idealized EPA test gas mileage numbers, but that in ordinary driving, the results are not so great. The turbo is never actually turned off, it just contributes more or less at various RPMs. The turbocharging forces the manufacturer to adjust the air/fuel mixture to be less efficient, in order to avoid knock. So, the manufacturer may be essentially manipulating things to look good on the EPA tests.  A larger engine, where some of the cylinders are shut off when not under load, may be more efficient. See video.