By Rud Istvan,
This possible guest post was inspired by Andy May’s recent post concerning mainly internal engine combustion (ICE) alternatives. Since I am a SME in the general field (including several basic supercapacitor patents), thought I would provide some factual engineering perspectives to WUWT.
First, combustion engines come in two basic forms: internal, and external. External (1819, Sterling) was never proven practical despite Dean Kamen’s fairly recent trying to get me as head of Motorola strategy and innovation at the time to invest. His fundamental engineering problem was simple. In ICE, most of the necessary thermodynamic cooling exhaust heat leaves via the tailpipe. The remainder (about 20%) is in the big car radiator. In a Sterling ECE, ALL the working exhaust heat must leave via a REALLY BIG radiator. NOPE.
Second, ICE engines come in various stroke flavors. For purposes of this simplified discussion, just two fours: Otto cycle and Atkinson cycle. Otto cycle is your ordinary 4 stroke piston car. A compression fuel upstroke, a combustion downstroke, an exhaust upstroke, then an intake fuel downstroke.
An Atkinson cycle uses the same four strokes slightly differently. The compression upstroke air intake varies (via complicated valve timing). So delivers more fuel efficiency (about 15%) but less torque efficiency on the combustion downstroke. (Oversimplified explanation: more uptake air, less fuel, less combustion downstroke torque.)
In what follows, we learn full hybrids can fully compensate for that Atkinson cycle fuel efficient torque deficiency.
There are several flavors:
- Full (Prius)
- Plug in (New Prius, Chevy Volt)
We will define all, but only consider to any extent full and plug in.
- Mild hybrids basically just do engine off at idle. NOT simple with conventional automatic transmissions, which is why FORD went with all DCT by 2018. Now this also (as BMW learned) still kills SLA battery life despite DCT. Turns out the necessary additional AH battery sizing (even at low PbA battery cost) “killed” that simple’s application. Valeo’s system is just one ‘dead’ example.
- Moderate hybrids add regen braking. The problem is, unless a really big battery, regen kills PbA even if PbA is oversized for starts) battery life by ‘overcharging’. Kills that application also.
- Full hybrids work, as posted here previously. Downsize the ICE (mine is a small I4 Atkinson cycle), make up the torque loss with the electric machine. Idle off at stop is free, and regen braking is free. For comparison, the HP and towing 2007 equivalent 4WD Ford Escape V6 got about 20 MPG average, our full hybrid equivalent gets about 30. Plus, we use regular, the v6 equivalent used hitest. About a dollar a gallon difference in these parts, and about 1/3 less gallons for a HP and towing equivalent small SUV.
- Plug ins. (like Chevy Volt and new plug in Prius). These by definition have larger, more expensive, and heavier batteries, yet still have all the heavy range extending backup ICE equipment. At the present (unnecessarily elevated) price of gas, still a very bad economic tradeoff. And, there is a hidden subtly. In full hybrids, the traction battery floats between about 60% and 40% charge. Nevermore, never less. That maximizes its life by design.
In a plug in, you drain the battery until the charging engine cuts in. That guarantees a much shorter battery life. Not a good thing economically.