What is pulse and glide?
Pulse and glide is a fuel saving technique used primarily by hybrid vehicle hyper-milers, but after driving a hybrid for a while you can make it normal driving habit and its completely safe too (unlike other techniques used by non-hybrid vehicle hyper-milers).
How does it work?
It works by keeping the engine on only whilst it is doing useful work in its efficiency range. Outside of this, such as minimal acceleration to maintain a set speed, is wasteful because it is operating in an inefficient power range. Put simply, the engine is using more fuel to keep itself running and overcome internal friction and compression than fuel used producing useful propulsion.
Here is how it works in bullet points:
- Use the engine at its efficient RPM (in my case 2000-2500rpm) to produce useful propulsion.
- Get up to just above your desired speed, then let off the accelerator so that the engine can stop.
- Accelerate just enough to overcome regenerative braking, but don’t use the electric portion of propulsion. Keep it as close to zero acceleration and zero braking, as if you were in neutral.
- When your speed drops too low, accelerate again up to just above your desired speed, and repeat.
Why does it work?
Here are some reasons why it works:
- Manual and automatic transmission vehicles have engine drag or use fuel to idle when you come off the accelerator or put it into neutral.
- In a hybrid the engine can stop because it can decouple the transmission from the engine so that there is zero engine drag and zero fuel use when it turns off.
- In a hybrid, the battery power must come from somewhere, and that somewhere is the engine. You want the engine to only charge the battery whilst you’re using it for useful propulsion at the same time, keeping it in its efficiency range. Pulse and glide allows this because you avoid using excessive battery power, plus you can still recapture some energy used during the glide through regenerative braking as well as engine recharging during your next pulse.
What sort of results should you expect?
It will vary depending on the speed, your driving style, the terrain and a few other factors such as whether your engine is warm (so it is able to turn off, and when it is on is operating as efficiently as possible).
Lets assume for my example that its a sensible 30mph with a smooth driving style, relatively flat roads and a fully up to temperature engine:
I found that when not using pulse and glide I was depleting my hybrid battery from full to almost zero in the 1.5 miles where I am able to use electric power on my last leg of my journey home. I was using the engine to get to 30mph (as the electric acceleration is wasteful of battery and is very slow) and then using electric to maintain that speed. By the end of the 1.5 miles my battery was depleted.
With pulse and glide, my hybrid battery stayed almost completely full by the end of the 1.5 miles, and the engine was on for only another few seconds each time I was pulsing up to around 35mph. This offsets the several minutes the engine would need to run to re-charge the battery (whether under useful propulsion or not).
With pulse and glide my displayed MPG was not affected so much, mostly because the way the car calculates MPG doesn’t change between electric use and gliding (it assumes 99.9+MPG). But it will definitely increase it in the long term because you’re not having to use extra fuel to recharge the battery because you didn’t deplete it in the first place.
My use of pulse and glide is limited most days because of my engine being cold and because most of my driving is on motorways where pulse and glide doesn’t really work. However during around town driving the more you can use it the better the effects on your MPG.
It takes a little practice and definitely requires patience, but once you have mastered it you’ll see the benefits quite quickly. I imagine it will work best in summer though when the engine can get up to temperature quicker (and stay there longer). Using the engine for heat is the biggest downside to most hybrids as when the engine cools it must run to reheat itself regardless of your use of its power!