Old engine designs are new again

Jan. 1, 2020
Engineers constantly are challenged with designing automotive systems that perform better and last longer, while still costing less. Amazing advances have taken place in automotive design, and cars only seem to get better as time goes on. Part of thi

Today engines employ more Atkinson-like features, modernizing what was new decades ago.

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There isn't a whole lot of "resting on one's laurels" in the automotive industry these days. Engineers constantly are challenged with designing automotive systems that perform better and last longer, while still costing less. Amazing advances have taken place in automotive design, and cars only seem to get better as time goes on. Part of this dynamic can be attributed to an ever-tightening regulatory environment, but heightened expectations on the part of the consumer also play a major role.

Many of the improvements we enjoy in today's cars were enabled by the advent of computer control. Older, mechanically controlled systems have been improved vastly when placed under the direction of a computer. Some automotive concepts were not even practical until computers came along. Here's one example: The hybrid-electric vehicle (HEV) was conceived at the turn of the previous century, yet didn't become a reality until computers were able to take charge of the hybrid drive functions.

Our world has changed, and the internal combustion engine (ICE) is changing with it. The ICE has shown amazing resilience in the face of advancing technology. Many an expert has predicted its demise, but we are witnessing yet another resurgence in its vitality and relevance. Yes, much of this has to do with computer control of the various engine functions. However, now even some of the basics are changing. Let's take a look at where we have been, and then examine how some old ideas are revolutionizing the operation of today's automobile engine.

From Otto to Atkinson

The gasoline engine that we all know and love has been based on the Otto cycle, invented in 1876 by Nicolaus Otto (1832-1891). While others had proposed the four-stroke cycle idea before him, Otto was the first to make it practical. The Otto cycle is but one version of the heat engine, where heat energy is converted into mechanical work. Many other inventors in Otto's day were working on similar ideas, and some came up with designs that made their own place in history. Regardless, the basic ideas behind the Otto cycle gained traction over time and continue to this day.
Otto received patents on his invention, but these did not cover in-cylinder compression or the four-stroke cycle. This left the door open for other inventors to continue work on their own versions of the ICE. In 1882, British engineer James Atkinson introduced the Atkinson cycle.

Atkinson's design was based on the four-stroke cycle, but utilized a complex linkage system to connect the piston to the crankshaft. This resulted in one revolution of the crankshaft for four strokes of the piston (this is difficult to imagine in the absence of a working model). What was really important about Atkinson's invention, however, was that it allowed the length of the compression stroke to vary from that of the expansion (power) stroke. This gave the Atkinson cycle an edge in efficiency over Otto's design, because of a decrease in pumping losses and increased thermal efficiency. Unfortunately, the Atkinson cycle generated less torque, especially at the low end, and suffered from reduced peak power output.

What are Pumping Losses?

It takes power to make power. An internal combustion engine is an air pump, and the energy required to move air through the engine is known as pumping losses. The typical four-stroke cycle has one piston stroke that contributes energy to the system and three strokes that absorb energy. If the energy loss of the intake, compression and exhaust strokes can be limited (decrease in pumping losses), the overall efficiency of the engine can be increased.

Gasoline engines are at a disadvantage in regard to pumping losses because of their use of a throttle valve. It requires a good deal of energy to move air past the throttle valve during the intake stroke. There are a number of techniques to limit pumping losses, including late intake valve closing and use of exhaust gas recirculation (EGR).

Modern Applications

At the time, the Atkinson cycle didn't make much of a splash. Most of the focus stayed on the more versatile Otto cycle, which became a dominant technology when the ICE gained popularity in the early 1900s. This dominance continued for decades, as the Otto cycle's simplicity and power density proved to work very well for the automobiles of the time.

Starting in the early 1990s, a shift in focus was taking place in terms of personal mobility. The electric car was being revived after lying in dormancy for the better part of a century. But even this appeared to be a false start, until a compromise showed some staying power. Another old idea, the hybrid-electric vehicle, was introduced to American roads in 1999. Using the best of the internal combustion engine and the electric motor, the HEV demonstrated unparalleled fuel efficiency and became a symbol for the world environmental movement. Ten years later, sales have increased to the point where the HEV can be considered a mainstream technology.

The HEV has changed the role of the internal combustion engine. While the ICE is still the HEV's primary power source, it is now being assisted by an electric motor and is not required to produce torque across a broad rpm range. Otto's cycle has now lost some of its relevance, as engineers turn over every available stone looking for ways to improve efficiency. So let's dust off yet another old idea: the Atkinson cycle has an efficiency advantage over the Otto cycle. However, Atkinson's original design with the complex crankshaft linkage is not practical. Instead, the contemporary version of the Atkinson uses a late intake valve closing strategy to allow the compression stroke to be shorter than the expansion (power) stroke. This is similar to the Miller cycle that was invented in the 1940s, minus the supercharger.

The Miller Cycle

Another alternative heat cycle for internal combustion engines is the Miller cycle. Invented by an American engineer named Ralph Miller, the Miller cycle was designed to increase engine efficiency without an associated loss of output. This was accomplished by combining a late intake valve closing strategy with a positive-displacement supercharger. The Miller cycle was first used in large scale industrial engines, but also was introduced to the automotive world by Mazda as a 2.3 liter V6 in its Millenia sedan.

Late Intake Valve Closing

The key to the Atkinson cycle's efficiency is its ability to minimize pumping losses. During the intake stroke, the intake valve remains open after the piston has passed Bottom Dead Center (BDC) and doesn't close until the piston has risen part way through its compression stroke. During this period, air-fuel mixture is being pushed back into the intake manifold and on into another cylinder that is on its intake stroke. This effectively shortens the compression stroke and lowers the vacuum in the intake manifold, making it easier for the cylinders to take in an air-fuel charge. For a given engine output, the throttle valve needs to be open further as well, which reduces pumping losses.

The Atkinson's expansion (power) stroke is considerably longer than its effective compression stroke. The longer expansion stroke of the Atkinson cycle allows it to convert more of the heat energy into mechanical power, further improving the engine's efficiency. Another way of describing this is to compare the Atkinson cycle's compression ratio to its expansion ratio. The expansion ratio is the opposite of the compression ratio, and describes the cylinder volume at the end of the power stroke compared to its volume at the beginning. Ordinarily (as in the Otto cycle), the compression ratio is the same as the expansion ratio. However, the effective compression ratio of the Atkinson cycle is lower than its expansion ratio due to the late closing of the intake valve. This modification increases efficiency from 12 to 14 percent over a non-Atkinson engine.

Current Practice

The Atkinson cycle is being used extensively in current HEV designs. The easiest way to configure an ICE for the Atkinson cycle is to modify the intake cam. The intake cam profile can be easily changed on a DOHC engine to enable late intake valve closing. The simplest approach would use fixed timing on both intake and exhaust cams, so the ICE remains in the Atkinson cycle during all modes of operation. As in any HEV design, the reduced low-end torque of the ICE is compensated for by the use of electric motors.

The Toyota Prius HEV is now in its third generation with the introduction of the 2010 model. All three generations of the Prius utilize DOHC engines with variable valve timing (VVT-i) on the intake cam. With this approach, the intake cam can be advanced or retarded to switch in and out of the Atkinson cycle. This allows the engine to tune for power or efficiency, depending on vehicle requirements.

To maximize efficiency, simply retard the intake valve timing using the cam phaser. Conversely, the intake cam would be advanced to increase power output. Starting with the 2009 model year, the Ford Escape HEV incorporated variable valve timing on the intake camshaft (iVCT) in its 2.5 liter Duratec engine. This same Atkinson cycle design is also used on the newly introduced 2010 Ford Fusion HEV.

Looking Down the Road

The internal combustion engine will remain vital for some time to come, but its role is changing. As the ICE's job description narrows, we now view inventors such as James Atkinson in a whole new light. Atkinson's ideas lay dormant for over a century, but now have sprung to life as our expectations for the ICE have changed. Look for more "old ideas" to be put to work as the nature of our personal mobility changes in the coming years.

Tony Martin is an associate professor of automotive technology at the University of Alaska Southeast in Juneau, Alaska. He holds Canadian Interprovincial status as a Journeyman Heavy Duty Equipment Mechanic. He also has 18 ASE certifications, including CMAT, CMTT, L1 and L2.

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