Why are ethanol and LPG not so suitable for the current generation of hybrid cars?
One question that has been raised for quite a long time is the suitability of hybrid cars such as the Toyota Prius to alternative fuels. Even though the average Joe might already regard the lower gasoline consumption as a step forward in the efficiency field and other aspects such as reducing the dependency on petroleum from war-torn and other politically-unstable zones or some environmental concerns, the capability to run on other fuels either perceived as "cleaner" or more affordable due to regional availability is becoming more relevant. Ethanol and liquid petroleum gas (LPG, also labeled Autogas in some countries) are well-proven as automotive fuels in a worldwide basis, but the current generation of gasoline-electric hybrid cars has some specific operating conditions that impose difficulties to use those fuels.
There had been some talks about a gasoline/ethanol flexfuel Prius since its official introduction to the Brazilian market in 2012 while it was still in the 3rd generation, but it never turned into a production model. Because of the ethanol fuel specifications in Brazil, with a 96% concentration on volume and about 4% residual water content, vaporizing is not so easy during cold starts and thus some cold starting aid is required when the fuel is used in a conventional port-injection engine such as the 2ZR-FXE fitted to the Prius. Older dedicated-ethanol and earlier flexfuel cars in Brazil required an auxiliary gasoline tank for the cold starts, nowadays mostly replaced by a thermostatically-controlled electric heating system for the fuel injection rail. Due to the intermittent operation of the internal-combustion engine in a hybrid vehicle on heavy urban traffic conditions, ethanol ends up not being so practical for all-year round operation. Unlike vehicles with a conventional non-hybrid driveline, which have benefitted from the downsizing trend that brought direct injection and turbocharging into the mainstream car market, the majority of hybrid cars keep the naturally-aspirated port-injection layout even though it's not due to cost concerns as often perceived. Since the prevalent sales argument for hybrids is a direct comparison with Diesel in the field of overall energy efficiency and emissions, resorting to this setup allows a lower intake flow temperature which leads to a lower nitrogen oxides (NOx) emission. Due to a small portion of the air/fuel mixture always returning to the intake manifold during the compression stroke because of an intentional delay in the intake valve phasing to emulate the Atkinson effect that turns the power stroke longer than the effective compression stroke, more latent vaporizing heat is absorbed from the subsequent intake charges, allowing the engine to run leaner while mitigating the risk of detonation/pre-ignition and not increasing the NOx emissions.
Therefore, a lower heat rejection can be expected from a hybrid car's engine. Eventually, such residual heat could be recovered and applied to vaporize the fuel through a heat-exchanger like it's usually done in a car converted to run on LPG, however it doesn't eliminate the need to use gasoline during cold starts until the engine temperature stabilizes. However, an intentional production of residual heat meant to help vaporizing some alternative fuel might not justify from an overall efficiency standpoint, while a short run before the idle shut-off might not lead the engine to a temperature suitable for neither ethanol nor LPG without having to resort once again to gasoline for a smooth restart. An upgrade from port-injection to direct injection could overcome this issue, but the air intake temperature and the compression heating would become higher enough to reflect into an undesirable increase of NOx emissions that would cut one of the most important competitive advantages held by the gasoline-electric hybrids against Diesel in times like these post-Dieselgate days.
Hybrid car owners looking for some gasoline replacement are actually not totally deprived of options. Despite of relying on a heavier and bulkier fuel system that leads to some compromise of the loading ability, compressed natural gas (CNG) is suitable to the intermittent operation of the engine and, since it's already stored in vapour phase at basically every ambient temperature, cold start won't be an issue. For those who seek renewability when choosing an alternative fuel, biomethane is the way to go. Since the chemical properties are identical to fossil natural gas, a transition to biomethane is easy as most of the pipeline and refuelling infrastructure won't require major changes.