This article from the Center for Advanced Automotive Technology explains the differences in the hybrid types and how they can even be used together. If that is too detailed for you, the short difference between them:
In a series hybrid, the gas engine isn’t connected to the wheels, it just generates electricity to go to either an electric motor or a battery, to later go to the electric motor.
In a parallel hybrid, the gas engine and the electric engine both are connected to the wheels and software controls that provide the needed power at any given time. For the last 20 years, the parallel hybrid has been a moderate success at improving the mileage of gas engines. The Toyota Prius achieved about 50% higher fuel economy than the comparable Corolla for this time period, but at a substantial increase in initial purchase price. You need a modified gas engine, an electric motor, a battery, and software to blend it all together. Well worth the extra money if you drive a lot or gas prices are $4 a gallon, not really worth it at low gas prices or for infrequent drivers. These were much cheaper than plug-in hybrid vehicles (PHEV) I describe below when batteries cost a thousand dollars a kilowatt hour, but now that battery prices have dropped considerably, it doesn’t make sense to build a hybrid like this. Just add a 15 kWh battery pack and a charger for about $5,000 and your buyer gets a $7,500 tax credit, so the buyer gets a less expensive vehicle that has a better driving experience and lower costs. The traditional parallel hybrid has no advantage. No wonder that Prius sales have tanked since 2015 — many have gone to electric vehicles like the Tesla Model 3 or other vehicles.
Structure of a Combination Series and Parallel Hybrid. From Wikipedia, picture by Fred the Oyster, CC BY-SA 4.0.
The third type is a plug-in hybrid (PHEV). The most famous is the Chevy Volt, but since that is being discontinued, the Honda Clarity PHEV and the Toyota Prius Prime are prime examples now. They work similarly to the Prius but have a much larger battery (usually 10 to 20 times as large), and that means that instead of going a mile or so on electricity that is generated by braking, you can go 20 to 50 miles on electricity you got from home or a charging station.
Structure of a Series Hybrid. From Wikipedia, picture by Fred the Oyster, CC BY-SA 4.0 license.
The series hybrid looks overly complicated, it has to convert gas to motion, then to electricity, and then to motion again. My thought was that this can’t be a good solution. I was wrong. I missed a lot of things.
1. The reason for the inefficiency of the gas engine has a lot to do with varying speeds. That is one reason that you see so many cars with 10 speed transmissions. Gas engines are only efficient at certain speeds. If you just want some electricity out of the engine and you have a small battery, like if you need a boost for passing, you can design your engine to be a small, cheap generator.
2. Since most of the energy goes directly from the generator to the electric motor, it doesn’t lose efficiency going through the charger and the battery. Some energy goes to the charger and the battery, but not a significant amount.
3. The chart below shows all the cost savings of the Nissan E-Power vs not just a plug-in hybrid and fully battery electric car, but also a parallel hybrid. 10 speed or CVT transmissions are expensive and manufacturers like Nissan and Honda are figuring out that if you have an electric motor for low speeds, you can get away with a cheap single-speed transmission.
4. Reading reviews of the Nissan e-Power Note and the Honda Insight confirms that both manufacturers have been able to make these series hybrids for a very small premium over a regular car, and also not ruin the trunk space as was so common with early hybrids. The consumer surprise is the ability to use the technology not just for fuel efficiency (which, if it can add 50% to your MPG with only a small increase in cost, might be better than using a fancy turbo engine and 10 speed transmission to add a lot less MPG to your car). The consumer surprise is that it gives you the instant torque we all love from EVs.
I didn’t think you could do that with a small battery, but it appears that it’s possible — Toyota and other early hybrids just didn’t do it because it would hurt efficiency, a foolish decision. One of the huge advantages to electric cars is you get to have one car that is extremely efficient and extremely quick, even it can’t max out on both at the same time.
5. From a staffing perspective, you are building expertise in electric motors, batteries, and advanced software, and preparing your engine and transmission engineers for retirement. This is exactly what you need to do to prepare for an all-electric future, which many of us now realize is inevitable. As we covered here and others have covered here, Nissan will use e-Power not just in economy cars but also in its high-end Infiniti line.
6. From a marketing perspective, you don’t aim this at the early adopters. This is the technology you put in all the cars for the people who meet my 7 reasons to not buy a Tesla or other EV. Briefly, such consumers need off-road capability, towing, tons of cargo space, rural or apartment charging, or drive very few miles, have low income, or are risk averse. This is what you sell to the Laggards and Luddites.
7. Even when all your customers want electric vehicles at once, you can’t meet their demand because:
- You were too stupid to build battery factories or enter into contracts with battery manufacturers before the market got tight.
- You don’t want to lay off all your engine people or your government won’t let you.
- You don’t have the engineering talent to redesign all your cars and trucks at once, so you have to do it over a period of years.
The conclusion I have come to is that if you can drive your wheels with an electric motor — which you can do with an EV, a series hybrid, and sometimes a PHEV — the car will have instant torque, have good regenerative braking, be efficient, have low emissions, and be fun to drive. Gas, diesel, and traditional parallel hybrids can’t provide this experience, so are not going to meet people’s needs after they have experienced driving an EV.
Now, as battery costs drop, the pure EV is both the most enjoyable vehicle and the lowest cost, but at the present time, there are no low-cost, long-range EVs. Series hybrids offer the chance to have some of the benefits of EVs and still the long range of gas vehicles and the 5 minute fuel stops that most people are used to — at a very small premium to gas vehicles.
Now, that is all great if you live in Japan, Australia, or New Zealand, but what about those who live in other places where Nissan doesn’t sell the E-Power vehicles. Two answers come to mind: You can keep your eyes peeled for series hybrids coming to your market, or you can use one of the PHEVs available that can operate as a series hybrid. The tax credit in the US and many other places is available on PHEV vehicles like the Chevy Volt, Honda Clarity PHEV, BMW i3 REx, Toyota Prius Prime, Chrysler Pacifica PHEV, and Mitsubishi Outlander PHEV. You can use these vehicles in either EV mode or series hybrid mode, and the main disadvantage of PHEVs over series hybrids is the increased cost. This cost is largely balanced by the US federal tax credit.
If manufacturers are designing new vehicles, there is only one strategy that makes sense. Design as many full battery electric vehicles as you can supply batteries for and build series hybrids for everyone else. If building for a market that subsidizes PHEVs, build some more of those, but otherwise, EVs and series hybrids should meet all your needs and prepare you for the fully electric future that is coming faster than anyone imagined.