Engines - What's the difference?

Introduction

Over the last month, I’ve been doing a lot of research on cars.  It was time to upgrade our family mover to something with 3 rows.  Sounds like an easy feat, no?  Unfortunately, when you replace cars about once every 10 years, due diligence is required to ensure you’re getting the one that is best for your family.

So I set out on a quest.  First, I compiled a list of 39 SUVs and CUVs with three rows.  Everything from Mitsubishi Outlander starting at about $25,000, all the way up to the Tesla Model X starting at about $105,000.  I looked at every single one of those 39 vehicles.

One thing I quickly learned was that each of the companies had a very different idea of what makes a good engine.  Some opted for big V8s while some focused more on smaller forced induction 4 cylinder engines.  Why?  Is a V8 really more powerful that a straight 4?  What are the benefits of each design?

If this is something you are interested in too, then read on!

The Truth About the Number of Cylinders

The number of cylinders has absolutely nothing to do with power output.  Displacement does.  The bigger the displacement, the more power the engine will have (all other things the same), regardless of the number of cylinders that displacement is shared across.  There is also a consideration to be had for the shape of the combustion chamber, piston compression, ignition timing, and several other factors.

Look at all the super cars out there right now.  Acura NSX is only 6 cylinders.  The entire Porsche 911 line only has 6 cylinders.  Many of the new super cars all share the same principle, a bigger engine is not the ultimate goal.  And these are some of the fastest cars in the world.

The design of an engine is all about compromise.  There are a lot of factors that need to be considered that favour the ultimate purpose of the engine.  It must:

1) Have enough power to overcome the weight of the vehicle including itself,

2) Have a reasonable fuel consumption,

3) Balance out the vibrations caused by the moving parts,

4) Fit inside the vehicle.

So let’s talk about each of the individual layouts, starting from the smallest.

Inline 4 - Inline 4s are compact and easy to fit into small vehicles, but are not well balanced.  They often require an extra drive shaft to smooth out vibrations, but they are very efficient engines and can rev well without issue.  I4s are used when engine efficiency outweighs all-out performance.

Horizontally Opposed (Boxer Engines) - Horizontally opposed engines such as Porsches and Subarus are balanced and have near perfect behavior internally.  However, packaging is very difficult.  The engine needs to be mounted far out in front of the front wheels to make room for AWD, which is why Subaru vehicles have such terrible understeer. Porsches, because of their rear mounted engines, have the opposite problem where the engine is so far to the back that it causes terrible oversteer.

Inline 6 - Inline 6s are underused.  They out perform even V8s at power delivery and smoothness.  However, they are very long which makes fitting them in vehicles very difficult.  They also generally require rear wheel drive, since it’s nearly impossible to attach them to a drivetrain that powers the front wheels.

V6 - A V6 has a great size, as it is only 3 cylinders deep, and therefore fits in most medium sized vehicles.  However, there is a huge tradeoff with V6s.  They are very unbalanced and, in order to keep the engine’s longevity in mind, the engine must be tuned down a lot to prevent it from shaking itself apart.  The RPMs are also very heavily limited, resulting in a fairly low redline.  V6 engines are generally boring to drive when used in commercial vehicles.

V8 (Traditional Cross Plane) - A V8 engine is excellent at balancing the 4 engine requirements.  It is not great at any single one of them, but it is very balanced.

The cylinder configuration of a V8 allows it to be relatively smooth (vibrations are muted), and considering it is only 4 cylinders deep, it is relatively small and therefore fits in most larger vehicles.

The downside with a V8 is that the cross plane balance adds a fair bit of rotational mass, so RPM needs to be limited.

Flat Plane V8 - Flat plane V8s have lower rotational mass so they rev higher and generally produce more power, but lose some of the cross plane V8 engine’s balancing. It also leads to higher maintenance demands and shorter overall engine life.

V10 - Still no idea what the strengths of this engine are, other than being able to tell your friends you have something bigger than a V8.

V12 - The V12 is an excellent engine. It is effectively two inline 6 engines bolted together.  Very powerful, very smooth, but also very large and difficult to fit into a traditional vehicle.

Rotary - Rotary engines are not used very much except by people who like to work on their own engines, since they need to be rebuilt frequently.  However, they have extremely high redlines, and can stay at their redline all day, which makes them very exciting to drive.

What about Forced Induction?

Forced induction is when a compressor is used to compress the air to the engine’s intake,  thus increasing the compression ratio of the engine.  Because there is more air being fed into the engine, more fuel can be burned as well.  This can result in more power output from a smaller displacement engine.  There are generally two types of compressors: turbochargers superchargers.

Turbochargers rely on the volume and speed of the engine exhaust to spin up the turbine wheel.  The more exhaust coming out, the more compression the charger will have.  Because it therefore relies on the engine to produce exhaust before it can work, there is a delay in power called “turbo lag” so they are best used for high-rev performance.

Superchargers have virtually no lag time because the charger is always spinning proportionally to the engine speed, however they cause drag on the engine resulting in a loss in power and efficiency.  Many of the new engines that deploy superchargers use a clutch mechanism to detach from the engine when they aren’t needed.

The overall benefits of using compressors is that you can use a small engine to save fuel (when not in boost), but still have access to the power when it’s needed.  It is important to consider though, that depending on your driving habits, chargers can also result in worse fuel consumption that a larger displacement engine as well.  There is a much larger fuel economy differential between economical and spirited driving, than with naturally aspirated engines.

Electric and Hybrids

Hybrids are all the rage right now.  Virtually every manufacturer now is introducing a hybrid engine configuration.  Are these worth it?

Hybrids can have two focuses: Power or Economy

Supercar hybrids, like the Acura NSX and Ferrari LaFerrari, use the battery primarily to complement the gas engine to provide additional horsepower.  It still burns all the fuel of a traditional gas powered vehicle, but also uses the battery add more power to either the front or the rear wheels.  Also, battery power has their entire torque potential at 0 RPM, long before the gas engine can start producing power. This is a different use case, so I won’t talk about it here.

Economy focused hybrids are designed to reduce the load on the gas engine, thus reducing the fuel consumption.  There are a few problems with this.  Batteries of this caliber are expensive, heavy, and only last for a few years before they start losing their capacity and will need to be replaced.  Also, at a $10,000 or more premium, you will need to drive a LOT before you end up saving enough fuel to make it worthwhile.

Electric cars have made huge strides, especially since Tesla started pushing the advancement of the technology.  They are great for short trips, but fall short on long road trips, since the infrastructure is not yet in place to support quick charging, except in very select locations.

Conclusion

There is no perfect engine for every application.  When choosing an engine for your vehicle, you need to consider what your driving habits are, and the ultimate application for the vehicle being purchased.  But if power is your main concern, you need to look deeper than just engine size.